PHILIPPINE EAGLE Pithecophaga jefferyi BREEDING BIOLOGY, DIET,
BEHAVIOR, NEST CHARACTERISTICS, AND LONGEVITY
ESTIMATE IN MINDANAO ISLAND
A Thesis Presented to the Faculty of the Graduate School
Ateneo de Davao University
In Partial Fulfillment of the Requirements
for the Degree Masters of Science
in Biology (Zoology)
Jayson C. Ibaňez
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Without the assistance and cooperation of my colleagues at the Philippine Eagle
Foundation, I would not have completed my thesis. I am grateful to my research team
namely Mary Beal Jiloca, Camille Concepcion, Aniceto Allado, Adriano Oxales,
Giovanie Tampos, Maita Verdote, Medel Silvosa, Jo Cruz, Guiller Opiso, Joseph
Alcomendras and Perfecto Balicao. During the 1999 fieldwork, Donald Afan, Glen
Bueser and Kharina Bueser helped in data gathering. They were all very consistent and
passionate in observing the eagles in the wild. Dennis Salvador, my organization’s
Executive Director, has been very supportive and patient with me. Dr. Hector Miranda,
former PEF Science Director, colleague and a friend has been very influential in my
career and has inspired me to start observing breeding Philippine Eagles in 1999. I also
thank my co-managers, Domingo Tadena and Lito Cereno who were my mentors and
inspiration for my work with the Philippine Eagles.
I am grateful and very indebted to Prof. Fe Bagajo, my Thesis Adviser. She has
been an exceptional academic mentor whose continued support, inspiration, attention
and advising have been instrumental to my success at Ateneo de Davao University.
I am forever thankful to my colleague, friend and my wife, Bing Ibanez as she
was my field assistant during the 1999 field work, and more importantly has provided
support, friendship and inspiration. Dr. Nigel Collar was very generous to host my
internship at Cambridge University. He has provided intellectual stimulation and a
refuge that provided the right academic atmosphere for thesis writing. This thesis would
not have been written without his guidance. My research committee members: Dr.
Bernadette del Rosario, Dr. Cecile Clarinda Pasino, and Dr. J-ney Zapanta. They all
made important academic guidance and contributions. I also thank Dr. Todd Katzner for
the important comments over the manuscript.
I thank the local government units, Indigenous Peoples, and friends at Arakan,
Mati, Tarragona, Sibulan and Lantapan for providing permission and assistance to do
research within their locality.
Before I worked in Davao City, my undergraduate mentor Prof. Andy Dans
generously embraced me into his team and opened opportunities for work and further
learning for me. I am forever grateful to him and his family.
My enrollment to the Master’s Degree was supported by the Marubeni Energy
Services and the Philippine Eagle Foundation. Field expenses were supported by the
Marubeni Energy Services, the Peregrine Fund, the Critical Ecosystem Partnership
Fund, and the Philippine Eagle Foundation. The Department of Zoology, Cambridge
University financed my internship at Cambridge UK.
I also acknowledge the assistance of undergraduate students Mayette Sulapas
and Allen Conte. Thanks also to our field trustees who watched over the eagles and
their nests whenever researchers were away: Romeo Abe, Blackie Lomiston, Melanio
Dindin, Manolo Labonso, and Nene Balicao.
Support and inspiration were also provided by my family and friends and by
many colleagues in the conservation movement. To all of those people and
organizations that I’ve inadvertently omitted, thank you.
Finally, this thesis is for happiness, which in my case would mean Bing, Sofia
This study described the breeding biology, diet, behavior and nest characteristics
of Philippine Eagles in Mindanao using data from five Philippine Eagle pairs nesting
from 1999 to 2007. Using information on breeding success spanning three decades
(1977-2007), an estimate of longevity for Philippine Eagles was also calculated.
Although results for breeding behavior, diet analysis, and nests and nest tree
characterization did not vary considerably from previous studies, this study provided
additional details and insights from the parameters considered.
Data showed that the Philippine Eagle has the longest and energetically most
expensive parental investment for any birds of prey. Baseline values for nest
attendance, incubation and brooding bout duration, trip duration and feeding rates
during each stage of breeding for each sex were provided. Nest trees were located near
human communities and were at varying distances from the forest edge. Measurements
of nests and nest trees were not very different from previous nests, although surface
area of nests studied were larger on the average. Flying lemur Cynocephalus volans
was the primary prey species, and arboreal mammals was the most important prey
group. Because the populations of Mindanao arboreal mammals seem stable, these
animals will most likely remain as the important prey group on the island. At least 14
prey taxa were recorded and 9 were identified to the species level. Philippine Eagles
are opportunistic feeders that can shift their diet to what is available. When heavier,
native mammals are scarce, they seem to adjust by taking on smaller prey items at a
higher frequency. An explanation for sexual dimorphism in raptors predicts that female
take larger and heftier prey whereas the males on smaller, more agile prey. The study
found no evidence for prey base partitioning among sexes in Philippine Eagles. This
study documented the first evidence of predation on domestic animals and pets, and
two instances of breeding failures because of food stress. In order for adults to produce
sexually mature birds which will replace them when they die, each male and female
must live at least 29 to 30 years.
The fact that the Philippine Eagle is long-lived, invests a lot on breeding, and the
continuing habitat loss and persecution predict the Philippine Eagles’ vulnerability to
extinction. The species also nest along forest edges near upland communities and can
feed on domestic animals and pets. These make the eagles very susceptible to
shooting, trapping and other forms of human persecution. An egg and a nestling were
deserted most likely because of food stress. However, there are evidences that eagle
pairs in large, intact forests with stable numbers of native arboreal mammals are
breeding well. Because of the importance of increasing adult survival as well as
ensuring productivity, protection of breeding adults and the places where they nest are
important. Conservation of the home range where they hunt is equally important to
ensure that enough prey base is available. Community-based conservation (CBC)
approaches can be a potent tool for conservation in places where people and Philippine
Eagle conflicts are tense.
TABLE OF CONTENTS
TITLE PAGE i
APPROVAL SHEET ii
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
Background of the Study 1
Statement of the Problem 4
Significance of the Study 5
Scope and Delimitation of the Study 8
Definition of Terms
2 REVIEW OF RELATED LITERATURE AND STUDIES
Value of Conserving the Philippine Eagle 11
Conservation Status of the Philippine 11
Hatching and Fledging Success 12
Nesting Density and Population Estimate for
Entire Philippine Population Estimate and the
Breeding Biology and Ecology of Philippine Eagles 17
Breeding: Monogamy and Site Fidelity 17
Breeding: Eggs, Timing of Breeding, and
Nesting Behavior: Courtship and Pre-Egg Laying 20
Nesting Behavior: incubation and brooding 21
Growth and development of young 22
Diet and Feeding Regime 24
Predation on Domestic Animals 25
Nest Sites and Nest Trees
Description of Study Area 29
Field Data Collection 33
4 PRESENTATION, ANALYSIS AND INTERPRETATION
Nesting Phenology 38
Nest and Nest Tree Characteristics 39
Reproductive Success 43
Behavior of Adults 44
Pre egg-laying period 44
Courtship displays 44
Incubation stage 47
Sprig deliveries 47
Behavior on the nest 48
Prey delivery rate 50
Brooding stage 51
Sprig deliveries 51
Behavior on the nest 51
Prey delivery rate 53
Post-brooding stage 54
Sprig deliveries 54
Adult behavior on the nest 57
Prey delivery rate 57
Diel pattern of prey delivery 61
Parental care 63
Nestling Development and Behavior 64
Diet and Food Habits 66
Prey species and food habits 66
Philippine Eagle hunting techniques 70
Predation on domestic animals 71
Philippine Eagle diet and food habits at Apo and
Size difference between prey items hunted by male
and female eagles
Breeding failures as an effect of food stress 77
Estimates for Philippine Eagle adult longevity
5 SUMMARY, CONCLUSION, AND RECOMMENDATION
A Photos of Philippine Eagles, nests and/or nesting sites. 90
B Data sheet used in studying Philippine Eagle behavior on
C Philippine Eagle Ethogram. 96
D Sample of transmitter attachment and “window clipping”
method to mark Philippine Eagles
E Mean weight used for calculating biomass of prey items
and the references.
F Outcomes of breeding attempts by Philippine Eagle pairs
in Mindanao from 1977-2007.
G Calculation of longevity value for Philippine Eagles in the
wild state in Mindanao.
H 2008 weights of male and female Philippine Eagles at the
Philippine Eagle Center, Davao City.
I Mean weight of male and female birds in several moderate
to highly dimorphic birds of prey.
CURRICULUM VITAE 109
LIST OF TABLES
1 Local land uses within the five Philippine Eagle nesting
sites studied from 1999-2007.
2 Nest sites, season and breeding stages studied in
Mindanao from 1999-2007.
3 Actual (complete dates) and estimated (month)
occurrences of egg laying, hatching and first flight off
nest at four Philippine Eagle nests in Mindanao from
4 Physical characteristics of Philippine Eagle nests and
nest trees on Mindanao Island (all measures were in
5 Comparison of sex roles for the incubation stage during
daytime observations at three Philippine Eagle nests in
6 Nest attendance, mean trip duration, brooding bout
duration and standing bout duration during daytime
observations at Sinaka and Kitanglad nests in Mindanao.
7 Prey delivery rates during the brooding and post-
brooding (early and late) stages of the nesting period in
three Philippine Eagle nests in Mindanao.
8 Quantity of Philippine Eagle prey species observed
directly from courtship through the nestling stages at four
nest sites in Central and Eastern Mindanao, Philippines,
from 1999 to 2007.
9 Food-niche breadth (FNB), dietary overlap, and
estimated mean weights (g) of prey of Philippine Eagle
pairs at Kitanglad and Mt Apo during the nesting season
LIST OF FIGURES
1 Location of five (5) Philippine Eagle nesting sites studied
in Mindanao, Philippines.
2 Sprig delivery rate at Kitanglad and Mt Apo nests during
the post-brooding period. 18 = 18-27 days, etc.
3 Times of the day sprig was delivered at Kitanglad and Mt
Apo nests during the post-brooding period. 5 = 0500-
0559 hr, etc.
4 Comparison of the mean (n=3) for male, female and
combined male-female values for the number of prey
individuals delivered per day to similar measures for
each of three nests (Cabuaya, Apo, Kitanglad) in
5 Comparison of the mean (n=3) for male, female and
combined male-female values for average biomass
delivered per day to similar measures for each of three
nests (Cabuaya, Apo, Kitanglad) in Mindanao.
6 Diel pattern of prey delivery at 3 Philippine Eagle nests
during the post-brooding period in Mindanao. 6 = 0600–
0759 hrs, etc.
7 Diel pattern of delivery according to weight of prey at 3
Philippine Eagle nests during the post-brooding period in
Mindanao. 1 = 1 – 499 g, etc.
8 Percentage of time spent on diurnal activities by the
nestling during the post-brooding behavior at the
Cabuaya, Apo and Kitanglad nests in Mindanao.
9 Comparison between the prey mass categories brought
by male and female during the post-breeding phase in
three Philippine Eagle nests observed (Cabuaya, Mt
Apo, Kitanglad) in Mindanao. 1= 1–499 g, etc.
Background of the Study
What is man without the beast? If all the beasts are gone men would die from
great loneliness of spirit. For whatever happens to the beasts also happens to
man. All things are connected….Chief Seattle (Duwarnish Tribe).
If there is one statement that would explain the value of conserving
threatened species, this wisdom from an Indian Chief will suffice: people must
conserve wild animals for the benefit of humankind. But a lot of our unique
wildlife is facing extinction, not by natural causes but by the irresponsible human
exploitation of their kind and the natural places where they live. These creatures
at the brink of extinction include our national bird - the Great Philippine Eagle.
Called the “Haring Ibon” (English “King of Birds”), The Philippine Eagle
(Pithecophaga jefferyi) is a tall, huge, broad-winged tropical forest raptor
endemic to Luzon, Leyte, Samar and Mindanao islands of the Philippine
archipelago. As a long-lived, late maturing, slow breeder heavily persecuted in a
forest habitat that is rapidly disappearing, the Philippine Eagle is one of the
world’s rarest and most highly threatened bird species (Birdlife International,
2007). Birds of this species nest on large piles of sticks on epiphytes atop the
crown of big trees in a range of habitats from primary to residual forests near
human dwellings (Kennedy, 1985). Lowland dipterocarp forest is believed to be
the previous stronghold (Rabor, 1969; Kennedy, 1981; 1985), but recent
distribution records show that they occur in mountain forests as well (Collar et al.,
1999). The Philippine Eagle is the only species in its genus (i.e. monotypic,
Brown and Amadon, 1968), but owing to its massive size, ecology and
morphology, it was placed under the harpy eagle group (tribe Harpiinae) together
with the eagles of New Guinea Harpyopsis and South American Harpia and
Morphnus (Brown and Amadon, 1968). However, recent phylogenetic studies
showed that Philippine Eagles are closely related to snake eagles (Circatinae)
than to the three forest eagles mentioned earlier (Lerner and Mindel, 2005).
Conservation of the species was first brought to world attention in 1965
during a conference by the International Union for the Conservation of Nature
and Natural Resources (IUCN) in Bangkok, Thailand (Rabor, 1965). The late
Prof. Dioscoro Rabor alerted his foreign colleagues about the impending
extinction of the Philippine eagle, with only less than 100 individuals suspected in
the wild. Through the World Wildlife Fund, the Philippine government declared
the bird as protected a year later. Subsequently, a conservation program was
established. Since then, initiatives by the government and non-government
organizations to monitor the status and breeding success of known eagle pairs,
protect its remaining forest habitats, do conservation breeding of the species,
and implement public education programs grew in earnest.
Despite the growing public attention on the species, published information
on the biology and ecology of the Philippine Eagles remained few. The reasons
include fund limitations, the lack of investigators trained in research and scientific
writing, and the difficulty of doing research in often rugged and rebel-infested
areas (Salvador and Ibaňez, 2006). For example, since the last publication by Dr.
Robert S. Kennedy on the conservation status of the Philippine Eagle more than
two decades ago (Kennedy, 1981; 1985), only seven papers (3.5 papers in 10
years) were published in scientific journals. All of these were from Mindanao. In
contrast, no scientific paper has ever been published on eagle populations since
1985 on the islands of Luzon, Leyte and Samar. Because of this, the Philippine
eagle remains one of the least understood large raptors in the world.
As a critically endangered species that is facing a 50 % chance of getting
extinct in the next 30 years, the Great Philippine Eagle is a global priority for
conservation (Bildstein et al., 1998). But in order to save a species, its biological
and ecological requirements must first be understood - at least its basic survival
requirements – so that its well-being can be secured (Burnham and Cade, 1995).
It is difficult to save an animal that one does not understand well.
In 1999, the Philippine Eagle Foundation (PEF), a non-governmental
organization dedicated to the conservation of the Philippine Eagle and its
rainforest habitat embarked on a long-term study of the behavior and food habits
of breeding Philippine Eagles in Mindanao to address information gaps on the
ecology and behavior of the species. This thesis described the results for five
breeding pairs studied from 1999 to 2007.
Statement of the Problem
The breeding biology, diet and nest characteristics of Philippine Eagles in
Mindanao were partially described by Gonzales (1968) and Kennedy (1985). The
courtship and pre-egg laying period are yet to be documented. The activity
regimes of nesting pairs, including male and female patterns, were insufficiently
quantified. Prey species have been identified (Gonzales, 1968; Kennedy, 1985),
but only from a limited number of nests. There are unconfirmed local reports of
eagle predation on domestic animals. In the light of shooting and trapping of
Philippine Eagles as a result of perceptions that they are livelihood pests,
measuring the importance of domestic prey in the eagle’s diet is important.
Additional information on nests and nest trees must also be collected as forests
have shrunk for the past two decades.
This study aims to describe the breeding biology, diet, behavior, nest
characteristics and average longevity of the Philippine Eagle Pithecophaga
jefferyi in Mindanao Island using data from five nests observed from 1999 to
2007 and from historical data.
Specifically, it aims to:
1. Describe the nesting phenology of Philippine Eagles breeding in
2. Describe nest and nest tree characteristics,
3. Describe reproductive success,
4. Describe the behavior of adults during pre-egg laying, incubation,
brooding and post-brooding stages of the nesting cycle,
5. Describe nestling development and behavior,
6. Describe the diet and food habits of breeding adults and nestling, and
7. Estimate the longevity and productivity of breeding adults.
Significance of the Study
Preventing the extinction of species is a global priority and forms a pillar in
the world’s effort to maintain biodiversity (United Nations Environmental
Program/Convention on Biological Diversity, 2003). But one can not conserve
what one does not know and understand. Basic research on the biology and
ecology of threatened species, including knowing its ecological requirements and
the factors that limit its population in the wild, is therefore important for protecting
small populations and mitigating threats (Burnham and Cade, 1995). This study
is a response to a global call for more baseline research on this rare and
extremely threatened species as it was primarily designed to fill in the gap in
knowledge on the nesting biology and ecology of Philippine Eagles in Mindanao.
This study provided the first detailed study of the nesting biology, breeding
behavior, diet and nest characteristics of Philippine Eagles since those by
Kennedy (1981,1985) more than two decades ago. Knowing what Philippine
Eagles eat is important so prey species can be protected and enough of them
exist to meet the Philippine Eagle’s food requirements. Thus, knowing which
animals are the important food for the Philippine Eagles and later using this
information to design conservation measures that protect these prey animals and
their habitat are the direct benefits of this study.
As another aim of the diet study, evidences of domestic animals in the diet
of Philippine Eagles was also documented. Many eagles are being shot and
trapped by local people because they are seen to be pests of livestock.
Measuring the relative importance of domestic animals in the diet of Philippine
Eagles is important to properly address the growing conflict between local
communities and Philippine Eagles because of alleged eagle predation on
Getting baseline information on how eagles behave during the stages of
the nesting cycle, including the time and effort they allot for guarding, feeding and
caring for their young is also important to understand how these natural
behaviors can be maintained in the face of human impacts on forests habitats.
Helping the Philippine Eagles survive in a human-altered world is a big
conservation challenge. This study documented the behavior of five pairs only,
which is not enough database to fully understand how Philippine eagles behave
in forests under different levels of human influence. But researchers and
conservationists can build on the baseline data provided to better understand
how eagles behave in response to changes in its forest habitat.
This study is supported by the PEF, a non-government organization that
works on conserving Philippine Eagle populations. Baseline data will aid PEF’s
education and habitat protection projects. The results of the study were also used
as a basis in developing a species action plan for this endangered bird. The
Protected Areas and Wildlife Bureau (PAWB) of the Department of Environment
and Natural Resources (DENR) and the PEF convened a national workshop last
February 2008 and a 10-year framework of action to save the species from
further decline will be published soon.
It has been the government’s policy to involve stakeholders in the
conservation of species and habitats. A number of local government units in
eastern Mindanao have started delineating municipal forests as protected areas
within their jurisdiction using boundaries of Philippine Eagle territories. Local non-
governmental organizations are also using the Philippine Eagle as a flagship for
forest conservation and sustainable development in the uplands. All the
information generated will be useful for building better awareness and
appreciation of biodiversity among students and will hopefully encourage them to
take care of Philippine Eagles and other wildlife for sustainable development.
Scope and Delimitation of the Study
The breeding biology, diet, behavior and nest characteristics of Philippine
Eagles in Mindanao island was described and interpreted using data gathered
from five Philippine Eagle breeding pairs and nesting territories in Central and
Eastern Mindanao. These eagle pairs and their nests were studied from July
1999 to July 2007. Five pairs is not ideal. But with the rarity of the Philippine
Eagle, the fact that they occur in low densities in often rugged and inaccessible
terrain, the costs associated with doing behavioral studies, and the strict
government guidelines on studying critically endangered species in their natural
habitats, five pairs is a good number.
Data on breeding behavior comprised observations on activities during the
following stages: pre-egg laying or courtship stage, incubation, brooding, and
post-brooding. However, not all of these stages were observed for all of the pairs.
The nests were found at different stages of the nesting cycle (e.g. the Cabuaya
and Apo nests were found during the post-brooding period) and had different
breeding outcomes (e.g. nesting at Tarragona and Sinaka failed during the
incubation and the brooding stages, respectively). The post-fledging stage, the
last phase of the nesting cycle, will be excluded as there were not enough
resources to extend observation beyond the fledging stage. Data on diet was
based on the direct observation of prey items brought by breeding adults to the
nest. The longevity estimate used breeding success data for Mindanao alone
where substantial information from three decades of Philippine Eagle monitoring
by the PEF has been compiled.
Definition of Terms
Breeding Biology. Refers to the timing and the nature of the various stages in
the breeding cycle of a species. Among birds, these stages would be the
pre-egg laying, egg, incubation, hatching, nestling, post-brooding, and the
Conservation Status. Refers to the category of the species under
consideration according to the International Union for the Conservation of
Nature and Natural Resources (IUCN) classification and the efforts done
to conserve the species and its habitat
Critically Endangered. A species is Critically Endangered when it has a 50 %
chance of getting extinct in the next 30 years, and thus facing an
extremely high risk of extinction in the wild.
Breeding success. Breeding is successful when the young bird reached post-
Brooding stage. That stage in the breeding cycle of birds from hatching until
that period when the parents no longer sits on the young bird to regulate
its body temperature.
Congeners. Belonging to the same genus.
Diet. Refers to the species of prey consumed by breeding Philippine Eagles.
Inter-nest distance. The closest distance between active nests of two
monogamous eagle pairs.
Longevity. The average age by which an adult Philippine Eagle successfully
rears a young that reached sexual maturity.
Nesting phenology. Study of the timing of breeding and its various phases.
Nest characteristics. Refers to measures of eight nest and nest tree
Nest site fidelity. Refers to a trait of some species of birds in which they use
the same place for nesting and rearing young several times.
Philippine Eagle. A large species of a forest eagle that is found only in the
islands of Luzon, Leyte, Samar and Mindanao of the Philippine
Post-brooding stage. That stage wherein young birds on the nest can regulate
its own body temperature and thus, no longer depends on the warmth of
its parent’s body.
Post-fledging stage. That stage from the time the young bird flew off the nest
until the young disperses away from its parent’s territory and becomes
Pre-egg laying or courtship stage. That stage in the breeding cycle of birds
prior to egg-laying which broadly include nest building, courtship and the
REVIEW OF RELATED LITERATURE AND STUDIES
Value of Conserving the Philippine Eagle
Predators, like the Philippine Eagle, are considered good ecological
indicators because they are atop food chains and require wide home ranges
(Rodriguez-Estrella et al., 1998). Predatory birds or “raptors” are considered
good indicators of habitat quality because of their sensitivity to human
disturbance and environmental contaminants (Newton, 1979). In some cases,
raptors are used as an “umbrella species” for biodiversity conservation because
their protection results to the protection of all plant and animal species found
within their large home ranges (Burnham and Cade, 1995). More importantly, the
Philippine Eagle is a national patrimony that must be cared for. It is an apt
symbol of the Filipino people, being found only in the country and being the
country’s national bird.
Conservation Status of the Philippine Eagle
The conservation and management of any species is contingent upon an
understanding of its population dynamics – that is, how population number
changes through time as reflected in net death and birth rates (McGahan, 1968).
Fortunately, breeding records of several pairs in Mindanao from 1977 up to now
were amassed, providing insights on the success of eagle breeding in the wild, at
least in the Mindanao part of its range. For populations in Luzon, Leyte and
Samar where no nest or breeding has ever been studied, nothing is known about
breeding success rates there. Similarly, death rates are difficult to estimate as
the number of eagles dying in the wild remains undocumented. This is true in all
of the islands where it is found. But in Mindanao where several wild eagles were
turned over to and rehabilitated at the Philippine Eagle Center in Davao City, a
trend in the contribution of people to eagle mortalities can be inferred.
Hatching and Fledging Success
From 1978 to 2007, 81 nesting by 30 pairs hatched 73 young for an
average of 2.7 young hatched/year. Of those hatched 53 fledged, with an
average of 1.8 fledglings/ year. Overall, the breeding success in Mindanao from
1977 to 2006 appears to be 65 % or 0.32 young/ pair/ yr. In a previous summary
of breeding success of Philippine Eagles that excluded data from 1999 to 2007,
Mindanao Philippine Eagles registered an over-all success rate of 58 % or 0.29
young/ pair/ yr (Miranda et al., 2000). The average breeding success for large
eagles is less than one young/pair/year (Newton, 1979). This indicates that
Philippine eagles in Mindanao seem to be breeding within normal levels. Thus, it
seems that abnormal mortality rates among sub-adults and adults, and not
breeding failure, is the proximate cause of decline (Miranda et al., 2000).
The only source of information on deaths among wild Philippine Eagles
were from the outcomes of rescues and rehabilitation of birds captured by people
(Hinlo et al., in press). Out of 59 Philippine eagles that were admitted at the
Philippine Eagle Center (PEC) from 1970 to 2006, 36 birds died, 5 were released
back to the forest, while the rest were kept as breeding stock. Of the 36 birds that
have died, 16 (44 %) had an undetermined cause of death, 9 (25 %) died of
infectious disease, 6 (17 %) of metabolic and nutritional disease, 4 (11 %) died of
trauma, while one eagle died of cancer.
Although the primary causes of most of the deaths among eagles admitted
to the center are unknown, these cases are definitely human-caused, directly or
indirectly. Shooting and beating eagles that were trapped or cornered in the
forest are clearly human-inflicted, but so too are deaths resulting from prolonged
stress brought about by improper handling and feeding of trapped or caged
eagles by inexperienced captors. Looking at the timing of the deaths in 36 birds,
nearly half (16 birds) occurred within a month after admission. In all of these
birds, they were either too injured or mishandled to be saved, or too sick and
malnourished to be cured (Hinlo et al., in press).
Based on this figure, it seems that the minimum mortality rate due to direct
human interference is one eagle every year. But if we include the 19 eagles that
are alive and in captivity as mortalities as they are already removed from the
wild, it seems Filipinos are responsible for losing at least 3 birds in 2 years (1.5
eagles/ yr). But this is admittedly an underestimate as there are surely more
unreported cases of human-caused deaths, especially in remote areas of
Mindanao where news of eagles being shot or dying in captivity do not reach
Eagles can also be lost to natural causes such as old age and diseases,
but it is unknown how many eagles are dying from these causes. Of the
infectious diseases known to inflict eagles, Aspergillosis could be the most
important. Out of the 15 eagles whose causes of death at the PEC are uncertain,
10 were unconfirmed Aspergillosis cases. Caused by the fungus Aspergillus
fumigates, Aspergillosis is the most common cause of death in captive raptors in
Britain and the United States (Cooper, 1985). Golden eagles, gyrfalcons,
ospreys, goshawks and the rough-legged hawk are at high risk for this infection
(Joseph, 1998). And perhaps Philippine eagles too, as Aspergillosis were
responsible for many raptor deaths at the Center for Philippine Raptors in Luzon
(Celis R., pers. comm.). A. fumigates are ubiquitous, naturally occurring, and
often manifests itself as a sequel to some other forms of stress such as recent
change in management, poor condition and concurrent disease/injury (Redig,
Nesting density and population estimate for Mindanao
Current knowledge on the breeding density and estimate of population in
Mindanao is from the work by Bueser and colleagues (2001). By mapping the
nests of Philippine Eagles from 8 provinces in Mindanao from 1991 to 1998, the
density of adult breeding pairs was estimated at one pair every 127 - 133 km
Mean inter-nest distances was calculated at 12.7 km, while the average distance
between breeding areas (i.e. including pairs whose nests were not located ) was
Using the nest space average, Bueser and colleagues (2001) calculated
the amount of forest for each territorial space centered on nests using 1:250,000
land satellite interpretation maps (NAMRIA). The average extent of forest cover
for each nest area was estimated at 55 percent, or an average absolute value of
about 70 km
. The total forest cover for each nesting "space" was approximated
to include closed canopy (with more than 50% cover) forest, open canopy (with
less than 50 % cover) forest, montane and mossy forest. Old-growth forest
averaged 51.0 % for pairs with located nests (N = 13) and 56.0 % for all pairs (N
= 25), which includes those whose nests were not found.
To determine a population estimate, two forest cover averages were used:
a) 68 km
for pairs with located nests only, and b) 72 km
for all breeding pairs.
Using these densities and the estimated suitable forest area of approximately
, Bueser and colleagues (2001) estimated that there are about 201 ±
19 and 190 ± 23 occupied territories in the island. This estimate is conservative
since they excluded sub-adults and “floaters" which they said are difficult to
incorporate into the data set in the absence of information on survival rates and
dispersal. Implicit to this estimate is the assumption that all suitable areas are
Entire Philippine population estimate and the Luzon dilemma
Similar extrapolations on the population estimate to include the islands of
Luzon, Samar and Leyte suggest a total population of 340 pairs (Miranda, et al.
2001). However, the estimate for Luzon may have a large margin of error.
Extensive surveys during the late 1970s and early 1980s did not result in locating
an active nest (Kennedy, 1985). Although, 81 nestings by 30 pairs have been
monitored in Mindanao during the last two decades, no nest has been studied in
Luzon, despite the fact that the island has more forest cover than Mindanao. At
present, it is arguable whether this is a reflection of differential nesting densities,
forest type variation, biogeographical history of the population, or differences in
research and conservation efforts (Miranda et al., 2001).
Studies on Luzon populations are imperative for many reasons. Luzon
may have been separated from the Greater Mindanao (which include Samar,
Leyte, and Mindanao) by 10 million years (Heaney and Regalado, 1998). From
the standpoint of conservation genetics, a vicariant historical event could have
led to genetic differentiation between the Luzon and the Greater Mindanao
(Samar, Leyte, and Mindanao) populations. This will be tested in the near future
using molecular tools and should be an important consideration in management
decisions involving releases of captive bred or rehabilitated birds from one island
to another (Miranda, et al, 2001).
Breeding Biology and Ecology of Philippine Eagles
Studies on the breeding biology, behavior and diet of Philippine Eagles
are too few. In fact, the foundations of our current knowledge on the nesting
biology of the species still rely on only two studies - that by Gonzales (1968) who
observed a nesting pair and its young in Malalag, Davao del Sur from July 1963
to June 1964 and those by Kennedy (1981, 1985) who documented 5 nests in
different parts of Mindanao from December, 1977 to November, 1979. Data for
these are effectively summarized in the book Threatened Birds of the Philippines
by Collar and colleagues (Collar et al., 1999). Published notes and accounts of
previous investigators and results of similar studies on raptors outside the
country were also used to enrich the review below.
Breeding: Monogamy and Site Fidelity
It has been assumed that Philippine Eagles breed as monogamous pairs
(Gonzales, 1968; Kennedy, 1977; Kennedy, 1981). Incidentally, this is the most
common mating system among raptors (Newton, 1979). Monogamy is believed
to have probably evolved among birds to meet the parental care needs of the
young, among others (Newton, 1979; Mock, 1985).
Philippine Eagles are also believed to mate for life as long as the mate is
alive (Kennedy, 1981), but this has never been verified in the wild. No eagle pairs
have ever been marked, which is unfortunate because monitoring marked birds
has a lot of benefits, including tracking faithfulness to the same mate and
instances of mate replacement or mate turn-over rates (Young and Kochert,
1987). But eagles in captivity exhibit high mate fidelity. The longest time that a
captive eagle pair stayed together in one cage is at least 9 years (Tadena pers.
comm), and this pair still lives together. Meanwhile, some Bald eagle (Haliaeetus
leucocephalus) pairs which are about two thirds the size of the Philippine Eagle
can stay together in the same territory for 6 breeding seasons (Jenkins and
Jackman, 1993). For the Eurasian sparrowhawk Accipiter nixus which is about
one thirds the size of the Philippine Eagle, four years was the longest that a pair
was found on the same territory (Newton, 1979). Harvey et al. (1979) suggested
that long-lived birds and birds living in stable environments (e.g. tropical forests)
may be more faithful to mates than other birds.
There is no question that Philippine Eagles exhibit high fidelity to nesting
sites. For example, at Mount Apo in Mindanao, breeding has continued in one
core area for more than three decades now. The earliest record was in 1972
(Kennedy, 1977) and the latest during the 2005 season where a juvenile
successfully flew off the nest in July 2006 (Concepcion et al, in press). In Mount
Kitanglad, the first record of breeding was in 1986 (Miranda et al., 2000) and a
pair is still breeding up to now. There are 8 more pairs in Mindanao that also
nested at least twice in the same places (Miranda et al., 2001). Nest site fidelity
has also been observed in species that also exhibit mate fidelity such as Bald
eagles (Jenkins and Jackman, 1993) and in many other raptors such as the
Peregrine falcons, African eagles, Great Kestrels, Buteo species, and European
sparrowhawks (Newton, 1979). Nest sites that are used repeatedly are found to
be those places where breeding success is high so that re-use is advantageous
for maximizing raptor productivity (Rowley, 1983).
Breeding: eggs, timing of breeding, and incubation
Philippine Eagles lay only a single egg every other year (Kennedy, 1981).
Although early records say they can also lay two eggs (Grossman and Hamlet,
1964), it was only in two instances that this happened (Wylie, 1974). A captive
bird in San Diego Zoological Garden produced two eggs in 1974, one on April 4
and the other on April 5. The other instance was recorded in 1973 at the
Philadelphia Zoological Garden where one was laid on February 3 and the other
on February 17. Based on the monitoring of 28 breeding pairs in Mindanao
between 1978 and 1998, only a single egg was documented in each nesting
attempt (Miranda et al., 2001) which indicates that the previous records are more
of a rare exception than a rule. However, captive propagation of eagles
confirmed that eagles can indeed lay two eggs in one season, but only when the
first one was intentionally removed from the nest (Tadena, pers. comm.). Called
“double-clutching” method, this is being employed to increase the productivity of
captive birds. There are no recorded instances yet that double clutching occurs in
the wild. But if it does happen, it might only be when the egg is lost early in the
breeding season where the eagles could re-clutch. Otherwise, they would nest
anew the following year.
Based on five nests observed between 1977 and 1978, egg-laying in
Mindanao seems to occur from September to November (Kennedy, 1985).
Incubation lasts between 58 to 61 days (Gonzales, 1968; Kennedy, 1981). But in
captivity, incubation lasts 57 to 58 days only. Unlike in the wild where attending
adults rests in between incubation bouts, eggs in captivity are artificially
incubated for 24 hours non-stop (Tadena, pers. comm..).
Nesting Behavior: courtship and pre-egg laying
This stage in the nesting cycle is not adequately documented for
Philippine Eagles. There are only two studies on Philippine Eagle nesting biology
so that they can be described in detail and individually. Gonzales (1968) who
studied a breeding pair at Davao del Sur in 1973 briefly mentioned that the pair
made frequent daily visits to the nest and called more often seven days prior to
egg-laying. Kennedy (1985) saw more activities from a pair in Amabel, Mount
Apo which he observed from September 13 to October 6, 1980. He recorded
food exchanges from male to the female, copulations, collections of sprig or
branches, and vocalizations, all of which intensified in intensity and frequency
during the observation period. Unfortunately, they had to stop the study and
leave because of insurgency problems.
These observations, however limited, are consistent with the behaviors
associated with the courtship period among other raptors. Food exchange from
male to female, also called courtship feeding (see Lack, 1940) is typical when
egg laying season approaches and thus, is believed to help the female
accumulate enough body reserves for breeding (Newton, 1979). However,
courtship feeding is also suspected to increase the chances of copulation and
also might help in strengthening pair bond (Lack, 1940).
Besides the mechanical function of consolidating the nest, sprig or branch
collection can be a signal to advertise occupancy of a nesting territory, or a form
of sex solicitation display as most copulation occurs during bouts of nest building
(Newton, 1979). For greeneries, Newton (1979) suggests it could be a form of
sanitation or a way to maintain humidity in the nest. On the other hand,
Winberger (1984) thinks that nest greeneries may repel nestling ectoparasites.
Other courtship displays known for other raptors include elaborate aerial
displays, nest displays, greeting and solicitation ceremonies, pre-copulation
feeding displays, among others (Newton, 1979; Brown and Amadon, 1968). All of
these have yet to be documented for Philippine eagles.
Nesting Behavior: Incubation and Brooding
Philippine Eagles share the time for incubating the egg and caring for the
young, but the female spends more time and effort than the male. Gonzales
(1968) recorded that the male incubated the egg for less than one third of the
total incubation time recorded. However, he did most of the hunting during the
nesting. Kennedy (1981, 1985) also had the same results wherein the male did
most of the hunting from incubation until the first third of the nestling period, and
the female doing about two-thirds of the incubation (apparently also incubation at
night) and almost all of the chick-feeding duties during this period. Nest relief
ceremonies were also recorded by both investigators and are composed of the
male delivering food to the female off the nest and immediately taking on the
incubation duty while the female eagle feeds and grooms. When the female is
ready to return to the nest, she would call. Both studies also showed that the
young was brooded, protected and cared for until it was between 40 to 49 days
old (Kennedy, 1985; Gonzales, 1968). Thereafter, both the male and the female
hunted for food and share in delivering food to, and feeding, the young.
Growth and development of young
The young birds studied by Gonzales (1968) and Kennedy (1981, 1985)
varied in terms of the periods by which they showed patterns of behaviors.
Kennedy first observed the chick doing wing flapping exercises when it was 4
weeks old while Gonzales did not see this happening until the bird is six weeks
old. The young birds were observed out of the nest and exploring immediate
branches at 15 weeks and 16 weeks by Gonzales and Kennedy, respectively.
Gonzales missed describing this but Kennedy first saw the eagle feed by itself
when it was 54 days old. The young bird of Gonzales left the nest permanently
when it was 15 weeks. One bird observed by Kennedy left the nest when it was
18 weeks old while another when it was 23 weeks old.
The post-fledging period is when young eagles learn to fly and hone
hunting skills prior to independence and dispersal from its parent’s territory
(Brown and Amadon, 1968; Newton, 1979). Gonzales (1968), Kennedy (1985)
and relatively recent observations by Afan and his colleagues (2000) are our best
source of information for juvenile behavior during this period.
Both parents continue to provide food for the young during the post-
fledging period, but provisioning gradually wanes until the young becomes totally
independent. Kennedy (1985) first observed a successful kill of a young eagle
when it was 43 weeks old (11 months). Afan and his colleagues (2000) first saw
a young eagle grabbed a small unidentified prey from a tree knot hole when it
was about 12 months old. For the same bird, it was provided with food by the
parents at least 7 times from September 1998 to February 1999 (Afan et al.,
The post-fledging stage is also characterized by frequent flight exercises,
and other play behaviors. Afan and colleagues (2000) gave a detailed description
of a young bird’s flight repertoire which included gliding, flap-gliding, powered
flight, soaring and holding on to branches and vines upside down with wings
constantly flapping. The young bird was also seen taking branches and sticks
and “playing” with it as if it was a prey item being subdued or a material being
arranged on an imaginary nest bowl. All of these were interpreted as play
activities that improve flight techniques and hone hunting skills. Such play
activities happened in the absence of the parents, suggesting that hunting
techniques appear to be learned by the young independently. Young eagles
were mobbed by birds such as large-billed crows Corvus macrorynchus
(Gonzales, 1968; Afan et al., 2000) rufous hornbills Buceros hydrocorax
(Gonzales, 1968; Kennedy, 1985) and writhed hornbills (Afan et al., 2000), but
managed to stay safe sometimes with help from the parents (Gonzales, 1968).
The precise age the young becomes independent from its parents is yet
unknown as there has been no detailed and continuous study of the whole post-
fledging period in Philippine Eagles. Kennedy (1985), however, observed that a
day after a young eagle was last seen within the territory, the parent eagles
started copulating. They last saw the young eagle when it was 16 months and 22
days.. Whether or not the parents actively drive young eagles out of the territory
remains an open question. But in some large eagles like the crowned eagle in
east Africa, the young seem to leave on their own accord (Newton, 1979).
Diet and feeding regime
Our best source of information on the diet of Philippine eagles at least
during the breeding season is Kennedy (1981, 1985) with his study of the prey
species brought by 5 adult pairs to the nest in Mindanao. Fourteen vertebrate
prey items were recorded with flying lemur (Cynocephalus volans) as the most
frequent prey species delivered to the nest (54.2 % of identifiable prey). Palm
civet (Paradoxurus hermaphrodites) and Mindanao flying squirrel (Petinomys
crinitus) were the next most abundant prey items at a frequency of 12.0 % and
7.8 %, respectively. The rest were 3 species of birds, at least 3 species of snakes
and 2 species of lizards. The top five prey items brought to the nest according to
live weight were the young Philippine deer (Cervus mariannus) (13-14 kg), the
long-tailed macaque (Macaca fascicularis) (3-4 kg), palm civet (3-4 kg), Rufous
hornbill (Buceros hydrocorax) (1.8-2.3 kg) and the Flying Lemur (1-2 kg). In
contrast to the old generic name as the “monkey-eating” eagle, the species feeds
on monkeys (i.e. long-tailed macaques) infrequently.
Philippine eagles have a broad prey base and appear to be opportunistic
feeders (Kennedy, 1985), which is similar to at least two equally huge tropical
eagles in Africa and in central and south America. Mitani and colleagues (2001)
studied 2 pairs of the Crowned hawk-eagles Stephanoaetus coronatus (weight =
3.6-5.0 kg) in Kibale National Park, Uganda and they identified 16 vertebrate
prey species from prey remains, with primates being the primary diet, and bats,
rodents, hornbills and unidentified non-primate mammals and birds as minor
prey. The live weight of its prey items range between 1.0 to 14 kg. Rettig (1978)
studied nesting Harpy eagles Harpya hapija (7.5-9 kg) in Guyana and found the
same prey size variety. Harpy eagles fed on 15 vertebrate taxa and took animals
with live weights between 2.7 to 9.0 kg. All of the prey items though were
mammals. All these giant eagles share a preference for arboreal mammals (i.e.
>60 % of prey): flying lemurs for the Philippine eagle, primates for the Crowned
hawk-eagles and sloths and primates for Harpy eagles. All also exhibit the same
ability to prey on animals much heavier than themselves.
Predation on domestic animals
Local reports that Philippine Eagles take domestic animals as prey are
common in almost every place where they are found. They allegedly take
domestic pigs, young goats, puppies, cats and chickens, especially during the
nesting season. But this has never been verified. The earliest record of local
accounts is those by John Whitehead in 1896 who wrote in his field notes that it
is “well known to the natives as a robber of their poultry and small pigs”
(Whitehead, 1899). However, in previous observations of several nests in
Mindanao, no domestic animals were ever brought to the nest by the attending
adults (Gonzales, 1968; Kennedy, 1981; 1985).
Raptor predation on domestic animals is well documented and is the
subject of mitigating actions by local governments in US and Europe. For
example, Golden eagles (Aquila chrysaetos) allegedly fed on lambs (Ovis aries)
and a calf (Tigner and Larson, 1981; Wood, 1946), Bald eagles (Haliaeetus
leucocephalus) on sheep (McEneaney and Jenkins, 1983), the Black vulture on
sheep and young pigs (Sus scrofa) (Avery and Cummings, 2007; Lovell, 1947),
and the red-tailed hawk Buteo jamaicensis on sheep (Barney, 1959). Because of
these, raptors have been seen as nuisance and some resort to shooting and
trapping to get rid of them (Kenward, 1999). In aid of management, domestic
predation was studied in detail (Phillips and Blom, 1988; Davies, 1999) and
recommendations were provided to prevent further losses (Matchett and O'Gara,
1987; Kenward, 1999).
Philippine Eagles can take domestic animals as prey. When Philippine
eagles were trapped for radio-telemetry from 1998 to 2001 for example, domestic
rabbits were used as baits to lure them. Out of 6 eagles that were fitted with
transmitters, 4 were trapped using domestic rabbit (Oryctolagus sp.) as bait
(Miranda and Ibaňez, 2006). Eagles feed on palm civets, wild pig, and a young
deer, which closely resemble domestic cats, backyard pigs, and young goats,
respectively. Captive eagles at the Philippine Eagle Center are fed with meat
from goats, chickens, rabbits, Guinea pigs, and quails and they seem to breed
successfully with this diet. Philippine eagles, therefore, appears very much
capable of consuming domestic animals so the issue then appears to be not
whether they kill domestic animals but to what extent does this occur and under
Nest sites and nest trees
In all recorded instances, from published description and from personal
observations, nests are typically located in the crown of huge trees on the major
forking at the end of the trunk or along large horizontal branches. Nests
consisted of many sticks on huge epiphytic ferns, on large orchids or on a small
bed of ferns (Kennedy, 1985). Kennedy measured two nests and each had a
surface area of 1.13 x 2.74 m and 1.16 x 1.31 m, respectively. Kennedy also
measured two trees and each had a height of 46.0 and 44.7 m, respectively. The
nests observed by Kennedy were located in a range of habitats from virgin to
Formerly known to be dependent only on mature forests (Rabor, 1968),
nests found in the last two decades were in a variety of habitats ranging from
unlogged old growth forests, to logged forests, a mature agro forest and a forest
plantation. In a study of the breeding density of Philippine Eagles in Mindanao, a
total of 13 nests were visited (Bueser et al., 2001). And out of this, 3 nests were
in primary dipterocarp forest while ten pairs were found on mature secondary
growth dipterocarp forest. Four pairs nested in the forest interior while nine pairs
built their nest less than 100 meters closer to forest edges. Surrounding and
within the eagle areas are mosaics of dipterocarp forest at various states of
degradation and human-induced modifications characterized by agricultural
lands, agroforestry and grassland. Physical parameters of these 13 nests
however, were not measured.
Description of the Study Area
Five nests were studied in Mindanao between 1999-2007 breeding
seasons as part of the PEF’s Philippine eagle monitoring program. Two nests
were along the Eastern Mindanao Biodiversity Corridor, two were within
protected areas, and one was in a small forest fragment (Fig. 1). Climate in all
nests except one falls under a “Type IV” rainfall pattern wherein rains are
typically distributed evenly throughout the year. Mt Kitanglad has a “Type III”
rainfall pattern wherein November until April is typically dry while the rest of the
months are wet. Mt Kitanglad had a temperature range of 10.5–27˚C, and an
average annual rainfall of 3,800 mm (Heaney et al., 2006). These temperature
and rainfall data are probably typical for all mountains in Mindanao. The nest
sites are near human dwellings and the nature of land use and extent varied
(Table 1). Each nest site is described below (see Appendix A for photos).
Mt Sinaka, Arakan, Cotabato Province. Mt Sinaka (7° 22.68’ N, 125°
12.83 E) is an isolated, mid-elevation mountain range (peak at 1448 m) covered
by 19 km
of forests. Only about 5% of its forest is lowland (0-800 m) and most of
it is modified by agro-forestry practices. About 85% is montane forest (800-1,200
m) and the rest is mossy forest. Balobo (Diplodiscus paniculatus) and Lamio
(Dracontomelon edule) dominate the nesting area, with Malugay (Pometia
pinnata) and White Lauan (Shorea contorta) making up a minor component. The
nest tree was a White Lauan. Patterns of human land use include shifting
cultivation for cash crops, maintenance of coffee and fruit trees and, recently,
palm oil cultivation.
Local land uses within the five Philippine Eagle nesting sites studied from 1999-
Sinaka, Arakan Agroforestry (e.g. shade coffee, fruit tree propagation),
slash and burn farming
Cabuaya, Mati City Slash-and-burn farming, abaca collection, rattan
Apo, Davao City Slash-and-burn farming, shade coffee, abaca collection,
Tubaon, Tarragona Slash-and-burn farming, abaca collection, hunting, rattan
Kitanglad, Lantapan Experimental pine plantation, wildlife hunting
Cabuaya, Mati City. The nest (06
32.63' N " 126
1.60’E) is on a low
ridge in a lowland dipterocarp forest facing the east coast of Mt Hamiguitan. Mt
Hamiguitan is a mid-elevation mountain range (peak 1,680 m) covering about
of dipterocarp, montane, mossy and serpentine forests. Vegetation at
the nest site is secondary growth dipterocarp forest, which was subjected to
selective logging in the past. Decaying logs and tree stumps provide evidence of
these previous logging activities. Large clearings were also present as a result of
slash-and-burn agriculture. The forest floor had a thin layer of dry leaf-litter with a
humus substrate, and large boulders interspersed the forest floor.
Mt Apo, Toril, Davao City. The nest (06
57.85’ N, 125
22.56’ E) is
located at the eastern border of a strict protection zone within the Mount Apo
Natural Park (peak at 2,954 m). Mount Apo is a protected area with a total land
area of 721 km
(Mallari et al 2001). The nest site is about 30 km south-west of
Davao City and is being proposed as a 1,430 ha Habitat Management Zone
within the park. The active nest, a tanguile (Shorea polysperma,
Dipterocarpaceae) was close to an abandoned farmland right at the forest edge.
Approximately 75% of this proposed zone is covered with both secondary and
primary dipterocarp forest.
Tubaon, Tarragona, Davao Oriental. The nest (N 07° 06.40’ and E 126°
20.87’) is confined above the Kamanamayan River, on a steep slope in a
mountain range that includes the hills of Piagkiawan, Kasumpo, Painguban,
Maningkohon, Pagbawian and Sayapo. The forests of Tarragona are within the
Mount Tagub-Kampalili key biodiversity area (KBA) with an estimated forest
cover of 1,699 km
. Vegetation is predominantly lowland dipterocarp forests
under varying successional stages. Patches of corn and coconut plantations are
found close to the forest edges. The nest is within a 1,200-ha Barangay Forest
Management Site and a 2,400-ha Community-Based Forest Management Site.
Mount Kitanglad, Lantapan, Bukidnon. The nest (N 08
3.74’, E 125
59.67’) is inside the Cinchona Forest Reservation along the southern borders of
the Mount Kitanglad Natural Park (total area = 312 km
, Mallari et al., 2001). The
nest tree is locally called “igem” (Dacrycarpus cumingii). Vegetation at the nest
site is undisturbed to moderately disturbed lower montane forest, mixed with
mature exotic pines planted experimentally three decades ago. Terrain ranges
from relatively flat to gently sloping, with high rising ridges and cliffs completely
absent. The nest is about 1000 m away from a road that cuts through the forest.
Field Data Collection
Five Philippine Eagle nesting attempts were studied at different stages of
the breeding season (Table 2). Two nests were located through a nest reward
scheme of finding eagle nests, while the rest were actively searched using “look-
and-see” and foot survey methods (Bibby et al., 1998). Adult behavior during pre
egg-laying and nest-building stage were recorded for the Sinaka nest, during
incubation at two nests (Sinaka and Tarragona), during the brooding at two nests
(Sinaka and Kitanglad), and during post-brooding at three nests (Cabuaya, Apo
and Kitanglad). A total of 2,577 hours of observations were spent.
Nest sites, season and breeding stages studied in Mindanao from 1999-2007.
Sinaka, Arakan 1999-2000 Pre-laying and nest-
building, Incubation, early
Cabuaya, Mati City 2004-2005 Post-brooding 103
Apo, Davao City 2005-2006 Post-brooding 916
2006-2007 Incubation 74
2006-2007 Incubation, brooding and
Observations for aerial and other courtship displays were done at Sinaka
from open ridges and hills using a 25-60 x field scope and 8 x 40 binoculars.
Nests were observed from hides on trees and on the ground at least 60 m away
using field scopes. Observation periods were typically from dawn to dusk except
in several instances of heavy fog and rain. At the most, two observers occupied
each hide and recorded behavioral data at an interval of 10 min (i.e.
instantaneous sampling, see Lehner, 1979) using standard data sheets (see
sample in Appendix B). A pre-existing list of Philippine Eagle behavioral units (i.e.
ethogram developed from observing both captive and wild eagles, see Ethogram
in Appendix C) was used. “Rare events” happening between the 10 min interval
were also recorded.
Philippine Eagles are sexually dimorphic-females being larger than the
males (Brown and Amadon 1968)-so that sexes were distinguished with a high
degree of confidence. Apart from the disparity in size, behavioral cues (i.e.
female doing most of the incubating, brooding and feeding tasks, male doing
mainly hunting during the incubation and brooding stages) helped in
differentiating the sexes. During post-brooding observations in which the female
helped out with hunting, familiarity with distinct features (e.g. general frame,
coloration, size of legs relative to body, etc), behavioral idiosyncrasies (e.g. male
at Sinaka relatively uneasy whenever on the nest and near the fledgling), and
size of each adult relative to the fledgling complemented the means to distinguish
males and females with high certainty. At the Sinaka nest, the female bird had a
radio transmitter and a “window” clipped on her left wing (Appendix D).
All prey items brought to the nest were identified to the lowest taxonomic
category possible. For every item brought, the following data were recorded: the
time and weather it was brought, which adult delivered it, body parts that were
missing, whether it was fully consumed, and whether prey remains were
removed from the nest.
All nests and nest trees were measured using eight parameters, with the
exception of the Sinaka nest which was measured using two parameters only.
“Nest height” was measured from the level of the rim to the ground. The “Nest
tree diameter” was measured at breast height using a diameter tape. A
clinometer was used to measure “Nest tree height” and a tape measure was
used to measure nest width, nest length, nest breadth, nest support branch
diameter and nest distance from trunk. The nest was climbed using standard tree
climbing and rappelling gear. Geographic locations of nests were taken using
hand-held Garmin Global Positioning System (GPS) and mapped using ARCGIS
Adult behavior patterns across the nesting phases were described using
the following information: 1) daily sprig delivery rates, 2) daily prey delivery rate,
3) nest attendance - percent of instantaneous samples where adult was
incubating and brooding, 4) trip duration – mean time the egg and the chick was
unattended, and 5) incubation and brooding bout duration – total time spent
incubating or brooding. A bout of standing near the chick during the brooding
stage was also measured. Diet was described by measuring the percentage
frequency of each prey taxa - the no. of individuals in each prey category/total
no. of prey in the sample, and the percent prey biomass – no. of individuals
multiplied by the mean weight of that prey/total prey biomass (see Marti, 1987).
Food niche parameters were also compared between two Philippine Eagle
pairs. Mean weight of prey (MWP) was estimated to compare average biomass
of prey for each pair. MWP was measured by multiplying the total number of
individuals for each prey category by the average weight for that category, and
then summing these totals and dividing the sum by the total number of prey
observed (Marti, 1987). Values for average weight came from either published
records for the prey taxa or from actual measurements of sample animals. For
prey species for which published weights were absent, weight records of animals
close in appearance and size were used (see Appendix E). Food niche breadths
(FNB) for each nest were also calculated. FNB was estimated using Levin’s
(1968) equation: FNB = 1/ΣPij
, where Pi is the proportion of the ith prey category
of species j (Marti, 1987). Dietary overlap (i.e. similarity in prey species
consumed) was measured between the two pairs using Pianka’s Index (O): O =
ΣPijPik/√ Σ Pij
where Pi is the proportion of the ith prey category of
species j and k (Marti, 1987).
To emphasize the importance of adult mortalities and its implication to
Philippine Eagle conservation, an estimate of the longevity and productivity value
for Philippine Eagles was attempted using Brown’s (1966) equation: Longevity
= 2/(N/A [SM
] + age at sexual maturity), where N/A = breeding success (N = no.
of fledged young, A=no. of attempts reaching egg-laying) and SM
= number of
young that reaches sexual maturity (considering 75 % mortality or 25 % survival).
The Philippine Eagle breeding information summarized in Miranda et al (2000)
was complemented with recent PEF data from 1998 to 2007 to calculate a
breeding success estimate for the species (see Appendix F). In the absence of
actual data for Philippine Eagles, published values for the percentage of young
that reach breeding age were taken from results of ringing Buzzards Buteo buteo
and Ospreys Pandion halieatus. The equation was adopted from Brown (1966)
who employed the formula for several tropical raptors in Kenya (Appendix G).
PRESENTATION, ANALYSIS AND INTERPRETATION OF DATA
A. Nesting Phenology
Egg-laying occurred between the months of November to January at four
nests (Table 3). It happened on November 1999 at Sinaka, and November 2006
at the Kitanglad nest if one used the 58 days incubation period and counted
back from when the egg was hatched (January 21). At Cabuaya, the chick’s age
was about 4 months old when the nest was found. The young was already full
grown then and was no longer brooded by the female. At the Apo nest, fieldwork
in January 15-16 and April 4-5, 2006 verified an egg and a three-week old chick,
respectively. Using these information, it was surmised that the eggs were laid at
the Cabuaya and Apo nests on December 2004 and January 2006, respectively.
The Tarragona nest was already with an egg when it was found. But since the
egg failed to hatch, it was difficult to estimate when the egg was laid.
The egg hatched in January 13 at the Sinaka nest after 58 days of
incubation, and in January 21 at the Kitanglad nest (Table 3). Using the 58-day
incubation period, hatching could have happened in February and March at
Cabuaya and Apo nests, respectively. The nestling period at the Kitanglad nest
lasted exactly 144 days (i.e. 4 months and 24 days). The young at Kitanglad,
Cabuaya, and Apo fledged in June 2007, July 2005 and August 2006,
Actual (complete dates) and estimated (month) occurrences of egg laying,
hatching and first flight off nest at four Philippine Eagle nests in Mindanao from
Egg laid Nov 16, 1999 Dec, 2004 Jan, 2006 Nov, 2006
Egg hatched Jan 13, 2000 Feb, 2005 Mar, 2006 Jan 21, 2007
First flight off
- Jul, 2005 Aug 2, 2006 Jun 15, 2007
If the previous record of nesting in September by Kennedy (1985) is
included, egg-laying appears to occur within a 5 month window, from September
to January. Using data from this study and previous data (Gonzales, 1968;
Kennedy, 1985), incubation lasts from 58-61 days. This is so far the longest
incubation period among raptors (Newton, 1979). Using previous and recent
data, the nestling stage can last between 105 – 163 days old. Post fledging
period according to Kennedy (1985), lasts until the eaglet is 640 days old.
B. Nest and nest tree characteristics
The five nests were at elevations between 630 – 1,434 m. The Kitanglad
nest is the highest in terms of altitude and is in a mid-montane forest that has
mature stands of exotic pines. In terms of forest cover, all except one were in
forests with size ≥30,000 km
. The exception was Sinaka with only 19 km
forest cover. Four species of trees were used namely White Lauan (Shorea
contorta, Sinaka), Red Lauan (Shorea negrosensis, Cabuaya ), Tanguile (Shorea
polysperma, Apo and Tarragona), and Igem (Dacrycarpus imbricatus, Kitanglad).
Nest trees were 35–60 m in height (ave. 46.1 ±13.4 m), and nests were 20–46 m
from the ground (Table 4). Nest trees were large, with DBH averaging 1.75 m. All
nests were on a horizontal branch 2–8 m away from the trunk. Nests averaged
2.25 m long and 1.4 m wide.
Physical characteristics of Philippine Eagle nests and nest trees on Mindanao
Island (all measurements were in meters, m).
0.91 35.40 - - - - -
2.32 65.63 1.48 2.40 0.52 1.07 2.94
2.34 43.75 1.12 2.27 0.65 0.68 1.90
1.38 37.81 1.90 2.76 1.20 1.80 8.13
1.77 36.88 1.09 1.56 0.44 0.47 1.67
1.75 43.89 1.40 2.25 0.70 1.00 3.66
0.62 12.56 0.38 0.50 0.34 0.58 3.03
NH-Nest Height, DBH - Nest tree diameter at breast height, TH-Tree Height, NW-Nest
Width, NL-Nest Length, NB-Nest Breadth (Thickness), NSBD-Nest support branch
diameter, NDT- Nest distance from trunk.
The five nests were in a range of habitats that were almost similar to the
places where Kennedy (1985) and Gonzales (1968) did their observations. The
nests were at varying distances from the forest edge, ranging from directly along
the edge to about 1,000 m inside the forest. None were in the immediate vicinity
of a primary forest. Two were in lowland forests (630–779 m altitude), while three
in mid-elevation forests (898-1,430 m altitude). All were logged selectively in the
past and now locally exploited either for agro-forestry, abaca hemp collection, or
Data for nest and nest tree parameters were not very different from
measurements made by Gonzales (1968) and Kennedy (1985), but with the
exception of measurement for nest surface area. The 1963 nest at Davao del Sur
had an estimated dimension of 1.7 × 1.4 m, while the four nests reported by
Kennedy had an average of 1.91 × 1.25 m. The five nests studied had an
average area of 2.25 × 1.4 m, which is slightly larger than previous nests. But
nest heights were within the known range. Gonzales’ estimate of 28 m and this
study’s average of 33 m fall well within Kennedy’s 27–46 m range. Nest trees
were equally huge with four located in steep slopes—two at the base, one mid-
way and one near a ridge—while one was in a gently rolling forest terrain. One
nest tree species Dacrycarpus cumingi, a non-dipterocarp tree, was previously
unrecorded (see summary of nesting tree species in Collar et al., 1999).
The eagle pairs studied here and those by Gonzales and Kennedy seem
to suggest a preference for forest edges as nesting sites. But such association
with edges may be an artifact of the ease of them being found and reported by
people. Or, it could be a product of the restricted distribution of suitable nesting
trees and substrates for Philippine Eagles. As an effect of massive logging of the
lowland forest during the 1960’s, most of the large trees are now confined at
forest edges in the mountains. Because eagles prefer large trees, they would
inevitably end up in forest edges if nesting substrates are limiting.
Suitable nesting sites are indeed limiting at least in two of the study areas.
Largely as a result of logging that peaked during the 1960s, the lowland forests
of Apo and Kitanglad have been depleted. What was left were residual
dipterocarp forests on steep, inaccessible slopes. In these mountains, eagle
preference for forest edges as nest sites seems inevitable. This therefore
underscores the importance of managing and conserving actual and potential
nest sites at forest edges to ensure eagle productivity.
The nest sites were in the vicinity of human settlements, but the Philippine
Eagles seem to tolerate human presence. They have repeatedly bred in these
sites despite its closeness to villages. The Apo pair seemed to have cohabited
with four communities living and farming inside the nesting territory. The earliest
record of nesting at Apo was 1977, and eight of the attempts resulted to a
fledged young (Miranda et al.,2000, this study). At Sinaka, the adult pair nested
six times since 1995 despite its closeness to two communities. These attempts
have resulted to four successful fledging (Miranda et al., 2000). The nests at
Cabuaya, Tarragona and Kitanglad are 0.5–1 km from the closest community.
Despite human presence in the forests, eagles were constantly detected in
Cabuaya during fieldwork that began in 1991 until the nest was finally located in
2005. At Tarragona, the pair re-nested on a new tree close to the 2007 nest at
the edge of a slash-and-burn farm. At Kitanglad, the 2007 nesting was the
second attempt after the territory was discovered in 2006. Strong fidelity to
nesting sites is typical among territorial raptors (Newton, 1979) and appears
extreme in Philippine eagles (Miranda et al., 2000; Collar et al., 1999).
Deforestation trends in Mindanao seemed to have confined several
Philippine Eagle nesting sites along forest edges so that several surviving pairs
seems to have no choice but to breed near upland communities along these
edges. The nesting phase is that part of the eagle’s life cycle whereby activities
of this rather wide-ranging species revolve around caring for a single young. It is
within these nest-bound stages of their lives that they become very vulnerable to
direct persecution by local people. Because Philippine Eagles exhibit extreme
fidelity to nest sites that are close to upland communities, conservation strategies
that sustainably meet the needs of the people without compromising the eagle’s
survival and breeding needs as well are therefore critical.
C. Reproductive Success
Three of the nesting attempts by five pairs from 1999 to 2007 resulted to a
fledged young, registering a success of 60% or 0.3 young per year. The 1999
attempt at Sinaka failed 9 days into the brooding stage, after three days of
intermittent rains and strong winds. The chick’s death, apparently because of
hypothermia, was confirmed when the female picked the carcass and ate it. For
the 2006 attempt at Tarragona, the female wandered away for almost two days
and left the egg unguarded. The male delivered only a single prey item to the
incubating female within eight days of observation. She left for a long time
apparently to feed. Unfortunately, while the female was away, a long-tailed
macaque snatched and crushed the egg. This is the first record of a nesting
failure caused by a marauding animal in the absence of an attending adult.
A success rate of 60 % or 0.3 young per year for the five pairs is not very
different from a success rate of 58 % or 0.29 young per year for nesting attempts
recorded from 1977 to 1998 in Mindanao (Miranda et al., 2000). These figures
are not also very different from success rate measured for breeding data from
1977 until 2008 (Appendix F), which is 66 % or .33 young per year. In stable
populations of large tropical eagles, an 80 % success or 0.40 young per year is
expected (Brown 1966). Based on this data, it seems that Philippine Eagle
breeding is not that bad. Thus, our data supports the conclusion of Miranda and
colleagues (2000) that breeding failure is not a major cause of decline among
Philippine Eagles; at least not yet.
D. Behavior of Adults
1. Pre egg-laying period
Courtship Displays. Increased aerial displays and frequent perching near
the nest marked the start of the breeding season. From July 7 to September 2
1999, 14 mutual soaring displays over the nesting territory were documented.
Such displays were characterized by a soaring flight, circling in the same or
opposite directions, with the male usually higher. From great heights, the female
performed a diagonal drop with the male trailing in pursuit (dive-chase). It then
ended with the male overtaking at great speed, or with the male presenting its
talons. In two occasions, the female flipped over and quickly presented its talons
as well. Such mutual talon presentation occurs in many large eagles (see Brown
and Amadon, 1969). The pair also made cruising flights over the territory with the
female in the lead. The male also performed advertisement displays, which
included calling loudly from prominent perches, and soaring or cruising over the
territory (territorial flights).
There were male display patterns which included swooping without
contact at the perched female (mock attack), calling vigorously in a low, nearly
horizontal posture while facing the female (pre-copulation display), and delivering
prey for the female to consume (courtship feeding). Courtship feeding happened
at a rate of 0.35 prey per day (n=31 days) and was accompanied by copulations.
The female did “food begs” towards the male which were all performed on the
nest and were almost identical to sex-solicitation displays by the male.
A form of mutual display which may be significant for pair bonding was
also recorded. The behavior consisted of repeated pecking at the bill, wings, tail
and other body parts of a mate. On one occasion, the male ran its bill along the
female’s wing feathers in a manner similar to preening. Labeled “allo-preening”,
this interaction occurred prior to and after mating, when nest building, or during
The first record of nest material delivery was on September 17 1999, two
months before the egg was laid, and it continued all through the pre-laying period
at a rate of 1.7 sprigs per day. The male brought more nest materials prior to
egg-laying (29 against 22 for the female). Behavior sequence for nest building
included (1) acquiring leafy twigs (fresh or dried) or dried sticks (twig without
leaves) from nearby trees, (2) depositing on nest, (3) cutting materials into pieces
and arranging, (4) pressing materials against nest bowl using the breast (breast
press), and (5) momentary brooding. The time spent at nest-building prior to egg-
laying averaged 5.70 ± 4.18 min (n=43).
Copulation occurred on nest and nearby perches and ensued with or
without courtship feeding. During courtship feeding (or supplemental feeding, see
Newton, 1979), the male brought prey to the nest and gave loud and prolonged
calls that change into short, excited, and high-pitched whines when the female is
close. Calls became vigorous and gained speed as the male mounted the
feeding female. Calls waned immediately after copulation. The male mounted the
female with its talons curled. If without food, the male exhibits a relatively
submissive but wary behavior. With vigorous whining and wings slightly drooping
(sex solicitation display), the male walks in a low horizontal posture towards the
female. If the female showed no rejecting behavior (i.e. standing erect), the male
mounted her. The female eagle remained silent and stooped during copulation.
Except for food solicitations, any behavior that suggested sex solicitation on the
part of the female was not seen.
Copulations were noted on 27 occasions (25 during courtship, 2 during
incubation) between September 17 and December 18 1999. It was seen twice in
a period of 30 minutes (n=3) and in one instance, it occurred three times in one
day. A copulation attempt from mounting till dismounting lasts for 18.83 seconds
on the average (SD=2.93, n=6).
Copulation among Philippine Eagles seems to be more than what is
actually necessary to fertilize an egg. It occurred in a span of three months,
which is similar to what was observed among Crowned Eagles Stephanoaetus
coronatus in Kenya whose breeding biology is similar to the Philippine Eagles
(Brown, 1966). Such high copulation frequencies are consistent with reports for
other raptors and thought to be more pronounced in raptors than any other bird
species (Newton, 1979; Pandolfi et al., 1998; Arroyo, 1999).
Several explanations have been proposed. High copulation frequencies
strengthen the pair-bond (Newton, 1979). High copulation rates among Bearded
Vultures Gepaetus barbatus were thought to buffer the potential risk of extra-pair
copulations when the female is away (Bertran and Margalinda, 1999). But such
risk is not as important to a monogamous species like the Philippine Eagle. It
could be an insurance policy so that when nesting fails early in the incubation,
the pair can re-nest immediately. There is no evidence for this yet, but it is known
that re-nesting happens at the latter part of the year and not immediately after a
failure (Miranda et al., 2000; this study). Copulations also occurred even during
the incubation period, from 18 days (Gonzales, 1968) to one month (this study).
Continued copulation even after the female’s fertile period is again thought as a
means to strengthen pair-bond (Holt et al., 1992).
2. Incubation stage
Sprig deliveries. Delivery of branches to the nests during incubation was
exclusively by females at the Tarragona and Kitanglad nests, whereas both
adults did it at the Sinaka nest, although the female did most (i.e. 19 of 22
deliveries). Sinaka nest had the highest sprig delivery rate (1.2 per day, n = 19
days) while nests at Kitanglad and Tarragona, had substantially lower rates: 0.3
per day (n = 6 days) and 0.2 per day (n = 6 days), respectively. Combined, the
average delivery rate for the three nests is 0.6 ± 0.6 sprigs per day.
Behavior on the nest. In all of the three nests, the female had a
substantial share of the daytime incubation duty (Table 5). Out of the total
instantaneous samples for each nest, the instances that the female was seen
sitting on the egg ranged from 73-83 % (average=82%, n=3). In two nests,
however, the male spent more time for each incubation bout. At Kitanglad, only
the female incubated the egg at night, but the male in Sinaka did so in three
occasions. Females made short trips, averaging 0.32 hr (SE=0.18, n=3) to
groom, defecate, collect sprigs, or interact with the male. The female did long
trips as well that averaged 2.42 hr (SE=1.89, n=3) during periods when the male
relieved her of incubation duties. For the females at the Tarragona and Kitanglad
nests, long trips were made even when the male was not around. The longest
time that the egg was left alone at Sinaka was 102 min, 73 min at Tarragona, and
111 min at Kitanglad (mean, 95.33; SD, 19.86). At Tarragona, the egg was
exposed to 15 min of rain when the female was away for 73 min.
The attending adults groomed, defecated, delivered sprigs, turned the
egg, or fed in between incubation bouts. Grooming (i.e. preening, sunbathing,
etc) was performed on either the nest tree or a nearby tree. Defecation was
always in a different tree. Collected fresh branches were torn into pieces and
deposited on the nest bowl. Before sitting, the egg was turned by arching the bill
ahead of the egg and gently shoving it backwards towards the belly. Egg turning
was seen at 29 instances between 0800-1600 hr at the Tarragona nests and was
almost solely done by the female (93%). Incubating adults tugged at the nesting
substrate and were seen dozing intermittently. During the incubation period,
there were no attempts by the female to hunt.
Comparison of sex roles for the Incubation stage during daytime observations at
three Philippine Eagle nests in Mindanao.
Behavior Sinaka (N=18) Tarragona (N=8) Kitanglad
Nest attendance (%)
Mean trip duration (hr)
4.59 (SD=1.72, n=8)
0.09 (SD=0.04, n=4)
21.04 (SD=3.82, n=5)
0.30 (SD=0.20, n=22)
1.08 (SD=0.10, n=3)
0.50 (SD=0.24, n=13)
1.60 (SD=0.35, n=2)
Inc. bout duration (hr)
1.43 (SD=0.81, n=70)
3.14 (SD=1.70, n=11)
1.00 (SD=0.87, n=33)
1.52 (SD=0.03, n=2)
0.81 (SD=0.39, n=9)
N = observation days, *male eagle attended nest only at the 8
last four days of incubation, b = male did not attend to the egg during 4
At the Tarragona and Sinaka nests, the male relieved the incubating
female even without food. It appeared that a desire to incubate by either adult is
signaled through vocalizations or by merely alighting near the nest. Mates seem
to relinquish incubation by flying off the nest. Nest relief appears simple, with one
bird walking into the nest bowl only when the other is already off. When inside
the nest bowl, the attending adult approached the egg slowly with its toes
clumped and claws bent inwards. During the whole observations when the
female was incubating, the male hunted and delivered prey to the female.
Prey delivery rate. Prey was delivered at a daily rate of 0.10, 0.25, and
0.71 at Sinaka, Taragona, and Kitanglad nests, respectively. It is remarkable to
note that despite the low food delivery rate at the Sinaka nest (0.10), the pair
successfully hatched an egg. At Tarragona, the egg was deserted. The Kitanglad
nest had the highest prey delivery rate and breeding resulted to a successful
It is suspected that the breeding failure at the Tarragona nest was a result
the male’s failure to provide enough food to the attending female. The male did
not bring prey for five consecutive days prior to egg desertion. The female made
loud food begs but there were no response from the male. It is therefore not
surprising that the female left and tried finding food for itself, to the peril of the
egg. A similar incident happened at a 1983 nest whereby the female abandoned
the egg and hunted when so little food was brought by the male (Lewis, 1983). In
contrast, a high prey delivery rate during the incubation and throughout the
nestling period (i.e., range = 0.71–0.61 prey per day) was sustained at the
Kitanglad nest. Among several pairs of the Crowned Eagle in Kenya (Brown,
1966), a prey delivery rate of 1 in every 3.3 days (0.30 prey per day) was
reported for the incubation period. This is lower than that recorded at the
Kitanglad nest. However, Crowned Eagles hunt larger prey items so that lower
prey delivery rates may be alright as long as hefty prey is delivered.
3. Brooding Stage
Sprig deliveries. For six consecutive days of observation of the Sinaka
nest, a total of 18 sprigs were delivered, registering a daily rate of 3 sprigs. The
female did most sprig deliveries (14 out of 18, or 78%). At the Kitanglad nest, the
female solely delivered sprigs at a rate of 1.2 sprigs per day, with 81% of the
sprigs (n=17 days) being fresh leafy branches collected from nearby trees. The
average delivery rate at the two nests was 2.1 ± 1.2 sprigs per day.
Behavior on the nest. At the Kitanglad nest, the time spent for each bout
of sitting on the chick was very variable (Table 5, range 7–355 min). Shorter
bouts and lower attendance rates were typical during fair weather, whereas
longer bouts and higher attendance happened during heavy rains and cold, foggy
weather. During one rainy day for example, the female sat on the chick for 355
min straight, emerging very drenched only when the rain stopped. Diurnal
brooding ceased when the chick was 24 days old, but the female continued
sitting on the chick at nighttime and stayed close during the day either on the
nest or at nearby perches apparently to guard the chick.
In both nests, diurnal and nocturnal brooding was solely by the female.
The average attendance rate during brooding at two nests was 55.5%, which is
lower than that for the incubation period (see Table 6). During brooding, the
female also undertook two types of trips out of the nest; one lasting considerably
less than an hour and another beyond an hour or more. Brief trips lasted 0.30 hr
(SE=0.18, n=2) on the average, and were typically associated with self
maintenance behaviors (e.g. grooming, sunbathing, defecation), sprig delivery
and feeding. Female trips >1 hr averaged 1.47 hr (SE=0.26, n=2), and seemed to
be associated with long breaks during a fine weather and probably hunting
attempts too when the male had not delivered prey for some time. At Mt. Sinaka,
for example, the female went out on a 3.82 hr excursion when the male has not
delivered prey for 2 days. She came back with muddy talons and a soiled breast.
At the Kitanglad nest, there were 17 instances where the female just stood
near the chick, and the time spent doing this varied (range = 9–54 min,
mean=0.35 hr, see Table 6). These bouts of standing coincided with sunny
periods of the day and appear to help with shading the chick from the sun’s rays.
In several instances, the female stretched its wings above the chick seemingly to
shade the chick. Aside from shading and brooding the chick, the female also took
the prey from the male and had the lead role in feeding the young.
Nest attendance, mean trip duration, brooding bout duration and standing bout
duration during daytime observations at Sinaka and Kitanglad nests in Mindanao.
Behavior Sinaka (N=5) Kitanglad (N=9)
Nest attendance (%) 61 50
Mean trip duration (hr)
0.17 (SD=0.19, n=12)
1.65 (SD=0.38, n=4)
0.42 (SD=0.29, n=47)
1.28 (SD=0.16, n=6)
Brooding bout (hr) 1.34 (SD=0.76, n=14) 1.14 (SD=1.43, n=31)
Standing bout (hr) - 0.35 (SD=0.28, n=17)
N= observation days, SD=Standard Deviation
Prey delivery rate. The prey delivery rate at the Sinaka nest was 0.50
prey per day, and the chick died because of exposure to very cold weather. In
contrast, a delivery rate of 0.69 per day at the Kitanglad nest, which was
sustained until the post-brooding period, resulted to a successful breeding.
In terms of the frequency of delivery, the male at Sinaka delivered prey
during the brooding period at a rate that is not very different from that at the
Kitanglad nest (0.50 vs 0.69 prey per day). However, in terms of the average
prey biomass delivered, the male at Sinaka did poorly. Within nine days after
hatching, the three prey items brought by the male, which consisted of two
rodents and a hind quarter of a flying lemur, only amounted to a rate of 269.17 g
per day (Table 7). In contrast, the male at the Kitanglad nest delivered prey at a
rate of 1,823.75 g per day. Hence, what the male at the Sinaka nest brought to
the nest were very much insufficient to satiate both the female and the chick.
The female at Sinaka left the nest for almost four hours in an apparent
attempt to hunt, exposing the chick to rains and strong winds. Whole broods of
Sparrow hawks Accipiter nisus in Scotland have suffered similar mortalities from
sudden downpours when the female was away hunting (Newton, 1979). All of
these underscore the importance of either frequent provisioning, if the prey items
involved are small, or deliveries of hefty prey at reasonable intervals to prevent
egg and chick abandonment during the critical stages of incubation and brooding
4. Post-brooding stage
Sprig deliveries. The first 67 days of the nestling period registered the
highest sprig delivery, at a rate of 1.1 sprigs per day (± 0.1, n=3) at the Kitanglad
nest, and all were made by the female. Deliveries waned during the latter part of
the post-brooding stage (Fig. 2). Almost all of the sprigs were fresh (85%, n=48),
with dried sprigs (7) brought only at the latter part of the post-brooding stage.
Sprig delivery stopped altogether when the eaglet was 107 days old (i.e. 3
months and 17 days). At the Cabuaya nest, sprig delivery was never seen during
the 13 days of observation. At this time, however the estimated age of the young
bird was four months and the adults were seen only on the nest during prey
delivery and feeding.
Fig. 2. Sprig delivery rate at Kitanglad and Apo nests during the post-brooding
period in Mindanao. 18 = 18-27 days, etc.
At Apo, the first two months of the nestling stage was missed. Using
photos of captive-bred chicks and by backtracking from when incubation and
nestling was first verified, it was estimated that the Apo eaglet was about 2
months old at the start of the study. Sprig delivery rates at Apo seemed lower
when compared with the same age bracket for the Kitanglad nest (Fig. 2). More
than three-quarters of the sprigs delivered were fresh (19, n=24) and the female
brought twice as many sprigs (67%, n=24) as the male. But unlike at Kitanglad,
sprig delivery at Apo continued all through out the nestling stage. In both nests,
however, sprig delivery was mostly made during early morning (i.e. 0500-0900
hr, Fig. 3).
Fig. 3. Times of the day sprig was delivered at Kitanglad and Apo nests during
the post-brooding period in Mindanao. 5 = 0500-0559 hr, etc.
It is interesting to note that the delivery of sprig to the nest continued from
the nest-building phase until the post-brooding phase. Because the delivery of
nest materials continued across the nesting season, investigators believe that
nest-building has other functions. Besides the mechanical function of
consolidating the nest, sprig deliveries are thought of as (1) a means of nest
sanitation by covering debris on the nest, (2) a signal to advertise occupancy of a
nesting territory, (3) and/or a means of maintaining humidity in the nest (as
discussed in Newton 1979). Another explanation is that the use of green plant
materials serves to repel ectoparasites harmful to the young (Wimberger, 1984).
For Philippine Eagles, the nest sanitation hypothesis appears weak,
because instead of covering uneaten prey remains, the parents remove them
from the nest. This is obviously a quicker and more effective way of nest
sanitation to avoid ant attacks, fly infestation, and the endoparasites they can
bring. The occupancy hypothesis is equally weak, as Philippine Eagle breeding
densities have been reported to be very low (70–130 km
/pair, see Bueser et al,
.2003) so that competition for territories might not be as intense as in other birds
of prey. Maintaining humidity is unlikely a reason as humidity is not a problem in
a tropical forest. The ectoparasite-repellent theory, in the absence of other
probable reasons, might be true for Philippine Eagles, and is a good subject for
further investigation. It would be interesting to know as well if eagles gather
sprigs only from specific trees. One can also study the rate of decay of sprigs
deposited to the nest bowl and see if these correlates to the frequency of sprig
Adult behavior on the nest. Eagle behavior was observed almost
throughout the post-brooding stage at the Kitanglad nest. At 39 days into the
nestling period, the female delivered its first hunt after a day of absence.
Although occasionally watched over from a distant perch, the chick was left alone
on the nest even when there were rains beginning 39 days. Nighttime brooding
stopped altogether as well. The chick was only attended to when fed. The female
also spent substantially more time out of sight presumably hunting, which was
the case also at the Apo and Cabuaya nests where the chicks were already old
when found. The male still transferred prey to the female, but when the female
was not around, the male fed the chick himself. Both the female and male
occasionally ate from the prey while they feed the young.
Prey delivery rates. Overall, a mean of 0.71 (SD = 0.1, n=3) prey item
per day of observation was delivered during the post-brooding phase. On a per
nest basis, the daily rate for the delivery of prey individuals was highest at the
Apo nest (0.81 animals/day), followed by the Cabuaya nest (0.69 animals/day),
and the Kitanglad nest (0.67 animals/day) (see Table 7). In Apo, there was one
case where three animals were delivered in one day (two by the male), 9
instances where prey was rationed by each adult on the same day (the female
doing so first on 6 occasions), and two instances where the male alone carried
prey twice in a single day. The shortest and longest delivery interval between
male and female was 48 min and 321 min, respectively (mean = 181.9 ± 110.1, n
= 9 obs). The mean gap for exclusively male deliveries is 349.5 min ± 30.4.
Multiple deliveries also occurred at the Kitanglad nest by the pair and the
male by itself, each on three occasions. The shortest and longest interval in male
and female delivery was 35 min and 445 min, respectively (mean = 286.7 ±
220.4, n = 3 obs). The shortest gap for exclusively male delivery was 60 min and
the longest was 244 min (mean = 135 ± 95.1, n = 3 obs). In contrast, no multiple
deliveries were observed at the Cabuaya nest.
Prey delivery rates during the brooding and post-brooding (early and late) stages
of the nesting period in three Philippine Eagle nests in Mindanao.
(≤ 28 days)
/ day (g)
However, if we look at the average biomass of prey delivered each day,
Cabuaya had the highest rate (1803.85 g per day) and Apo had the lowest
(924.86 g per day). As shown in Table 7, adults at the Cabuaya and Kitanglad
nests hunted heavier prey items across the post-brooding stage while adults at
the Apo nest took lighter prey items. At the Apo and Cabuaya nests, the female
delivered more prey individuals than the male, but in the Kitanglad nest, the male
substantially brought more than the female (Fig. 4). Looking at the average prey
biomass though, the male consistently brought heavier prey animals in all of the
three nests than the female (Fig. 5).
Fig. 4. Comparison of the mean (n=3) for male, female and combined male-
female values for the number of prey individuals delivered per day to similar
measures for each of three nests (Cabuaya, Apo, Kitanglad) in Mindanao.
Over-all, prey delivery rates at the Cabuaya, Apo and Kitanglad nests
during the post-brooding stage were relatively high (Table 7). At this stage, the
female had stopped brooding and guarding the young and was already helping
out with hunting. This is consistent with reports that food deliveries become more
frequent at the latter stages of the nestling period where the food demands of a
growing chick are greater (Newton, 1979). All attempts at the three nests resulted
in a successful fledging. Hence, an average prey delivery rate of 0.72 prey per
day and 1,823 g. per day in terms of biomass during the whole nestling period
can be the average, if not the minimum, to ensure a successful nesting attempt.
Fig. 5. Comparison of the mean (n=3) of male, female and average biomass
delivered per day, with similar values for each of the male, female, and combined
rates at each of the three Philippine Eagle nests observed (Cabuaya, Apo,
Kitanglad) in Mindanao.
Diel pattern of prey delivery. At the Cabuaya nest, prey delivery during
the post-brooding stage followed a strongly bimodal pattern throughout the day.
The largest peak in deliveries occurred at mid-morning (0800 to 1000 hr), with a
second peak in the afternoon (1400 to 1600 hr). A similar pattern was also
observed at the Kitanglad nest. At the Apo nest, however, prey delivery rose
steadily throughout the whole morning until early afternoon, decreasing in late
afternoon. Pooled data for the three nests retains a bimodal pattern in the timing
of prey deliveries to the nests (Fig. 6).
Fig. 6. Diel pattern of prey delivery at 3 Philippine Eagle nests (Cabuaya,
Kitanglad, Apo) during the post-brooding period in Mindanao. 6 = 0600–0759 hrs,
Looking at the diel pattern of delivery according to prey weight (Fig. 7), the
bimodal peaks in delivery remained evident. The peak of delivery of prey within
the 1-499 g range seems to occur between 0800-1000 hr, and waning thereafter.
Delivery of long-tailed macaques (i.e ≥4000 g) had two peaks, one between 0800
to 1000 hr and between 1200 to 1400 hr. For flying lemurs (i.e. 1000 g), more
were delivered during early morning and around noon. No prey items from the
2000 g category were brought to the nest. Over-all, prey items within the 500 g
category had the highest frequency.
Fig. 7. Diel pattern of delivery according to weight of prey at 3 Philippine Eagle
nests during the post-brooding period in Mindanao. 1 = 1 – 499 g, etc.
5. Parental Care
Philippine Eagles have the longest and energetically most expensive
parental investments for any birds of prey (see Newton 1979). They consistently
rear only a single young within two full years for an entire breeding cycle (Collar
et al., 1999). Courtship and nest-building until egg-laying lasted four months at
the Sinaka nest. Incubation lasted 58 days at the Sinaka nest and 60–61 days at
the Davao del Sur nest (Gonzales 1968). In captivity, eggs hatch after 56–57
days of artificial incubation (Tadena et al., 1999). Among large raptors, only the
Andean Condor incubates eggs for at most 58 days (Brown and Amadon 1968).
At the Kitanglad nest, the nestling period lasted 146 days, which is within the
range of 104–164 days as observed in previous studies (Gonzales 1968,
Kennedy 1985). Parents continued feeding the young until the next nesting
season. In case of breeding failures during the first quarter of the year, they re-
nest towards the end of the year. This occurred at the Kitanglad nest wherein the
chick studied was a product of a re-nest in December 2006 when an 8–9 month
old eaglet of the same pair was shot and removed from the nest site in
September 2006. At the Tarragona nest where breeding failed in January 2007 a
re-nest happened December of the same year. Several re-nesting after a failed
attempt were also documented in at least four nesting territories monitored in
Mindanao since 1977 (Miranda et al., 2000).
Male and female roles during the nesting season were consistent with
previous reports (Gonzales, 1968; Kennedy, 1985). The primary role of the male
is to acquire food during the incubation and nestling period. The male also
relieved the female from incubation duty and fed the chick as well, but only
rarely. The primary role of the female is to incubate the egg and brood, guard
and feed the nestling. Females never hunted during incubation, or if they did,
during long forays away from the nest, there were no recorded successes.
Parental care in Philippine Eagles, whereby males provide nourishment to the
female and the young, and commonly play very little role in incubation and
feeding the young, conforms to that seen in the Harpy Eagle Harpia haypyja, its
ecological equivalent in South America (Rettig 1978), and to other raptor species
as well (Newton 1979).
E. Nestling Development and Behavior
Almost the whole nestling period was documented at the Kitanglad nest.
Hatching was not actually seen, but was inferred when the female seemed to sit
lightly on the nest bowl on January 21. Feeding was first seen on the same day,
but it was not until two days later that the chick was seen. By day 4, the chick
began crawling about. By day 12, the chick could sit, stand and swallow larger
pieces of meat, including the metatarsals of a palm civet. By 24 days, the chick
had gained its daytime thermal independence and could already walk well
around the nest bowl, doing quick wing exercises and playing with sticks, leaves
and prey remains (i.e. object play). By 43 days, it could feed by itself although the
adults fed it throughout the nestling period. At 54 days its behavioral repertoire
had widened to include flight exercises, wing-slaps (i.e. single loud wing flaps)
and mock attacks at prey remains and sticks on the nest which intensified as the
. Percentage of time
spent on diurnal activities
by the nestling during the
post-brooding behavior at
the Cabuaya, Apo and
Kitanglad nests in
chick grew older. However, most of the day was spent on general maintenance
behavior, but mostly either just standing or prostrate (brooding or sleeping) (Fig.
8). Generally, older chicks at Cabuaya and Apo nests behaved similarly (Fig. 8).
Within 3 weeks before fledging at the Kitanglad nest, the nestling explored
adjacent branches. At this time, agility exercises such as running across the nest
and wing-flapping while jumping were frequent. The chick also did hop-flights to
and from nest-support branches. Similar excursions and hop-flights and
increased frequencies of flapping exercises were seen in the chicks at Apo and
Cabuaya. The nestling also grabbed prey from attending adults and called
vigorously at adults on sight. In one occasion at the Kitanglad nest, the chick was
very aggressive towards the male on the nest while it was feeding on the prey
delivered by the female. The male was forced out of the nest by the vigorous
calling and aggressive advances of the young. The chick flew out of the nest
when it was 146 days old.
F. Diet and Food Habits
1. Prey Species and food habits
Taking all five Philippine Eagle pairs together, we recorded a total of 151
prey items (107 identified to the species level) from pre egg-laying until the post-
brooding periods (Table 8). At least 14 taxa were recorded and 9 were identified
to the species level. Nine of the prey taxa were mammals, which comprised 91%
of identified prey items. The top two prey items in terms of percentage frequency
were the Philippine flying lemur Cynocephalus volans (29.8) and the Common
palm civet Paradoxurus hermaphrodites (13.9%). Both species are arboreal and
nocturnal. Another nocturnal and arboreal prey item is the Mindanao flying
squirrel Petinomys crinitus, and it ranked fourth in terms of percentage frequency
(5.3%). The Long-tailed macaque Macaca fascicularis, an arboreal mammal that
is active only during the day, was the third most important prey item (10.6%).
However, if we look at which prey species had the highest biomass contribution,
the long-tailed macaque (34.8%) substantially becomes the most important prey.
Quantity of Philippine Eagle prey species observed directly from courtship
through the nestling stages at four nest sites in Central and Eastern Mindanao,
Philippines, from 1999 to 2007.
Cynocephalus volans 45
Petinomys crinitus 8
Macaca fascicularis 17
Felis domesticus 2
Unidentified mammal 22
Unidentified rodent 13
Unidentified bat 6
Naja samarensis 1
Tropidolaemus wagleri 3
Unidentified snakes 3
Sarcops calvus 1
Gallus gallus domesticus 6
Total mammals 137
Total birds 7
Total reptiles 7
Almost all of the prey animals brought to the nest were decapitated and
dismembered. Typical for almost all of the mammalian prey, only the torso and
the hind-quarters were intact. All were apparently from fresh kills. In the Kitanglad
nest for example, a Philippine flying lemur was delivered to the nest alive, and in
the Apo nest, a gravid pit viper was still twisting and struggling against the adult’s
talons when it was fed to the chick. Almost all parts of the prey items brought to
the nest were consumed, including entrails and whole limbs. Fore and hind limbs
were swallowed whole by the adults and the eaglet as well when it was older.
Uneaten prey remains such as dried flying lemur skin and a leg of a palm civet
were removed from the nest.
Our observations on the prey delivered to nests revealed both similarities
and differences to those reported by previous investigators. Mammals dominated
the diet as has been found by previous studies (Gonzales, 1968; Kennedy, 1985)
and flying lemurs were similarly the most important prey species by frequency.
But in terms of biomass contribution, this study showed that long-tailed
macaques became the most important prey species. In contrast, macaques are a
minor prey at the Davao del Sur nest in 1963. Out of the 48 prey items recorded,
only three were monkeys (20% of the total biomass). Flying lemurs comprised
79% of the total biomass (Gonzales, 1968).
At three nests studied by Kennedy (1985), there were 19 times more flying
lemurs than macaques, making the only primate in the country again a very
minor prey. The fact that more of a particular prey is delivered to one nest but
fewer of it at a different nest only reinforced the previous hypothesis that
Philippine Eagles are opportunistic feeders, taking whatever is available and
easy to catch (Kennedy, 1985). Such ability to shift diet depending on prey
availability was also observed among Crowned Eagles and Harpy Eagles.
Crowned Eagles prefer primates in forest habitats, but hyraxes and duikers in
open savannas (Mitani et al., 2001). Harpy Eagles favor primates in one territory,
but sloths in another (Galetti and Carvalho, 2000).
The high proportion of nocturnal mammals indicates the importance of
arboreal and nocturnal animals in the Philippine Eagle diet. Flying lemurs are
gliding foliagivores. They forage by night and roost during the day either in tree
cavities or by gripping a trunk or a branch with its patagium cloaking its body
(Wischusen and Richmond, 1998). The common palm civet is also arboreal and
rests up in the trees during the day in hollows or shelter of entangled vines
(Shiroff, 2002). There is no information on the roosting habits of flying squirrels,
but they could be cavity roosters as well, like the rest of the flying squirrels (Fox
and Mulheisen, 1999). These species are found from sea level up to the
mountains, but they are particularly common in the mid-elevation to montane
forests (Heaney et al., 1998). Because they seem to be thriving well in a habitat
that is not under immediate threat, arboreal mammals will most likely remain as
an important component of Philippine Eagle prey base across Mindanao.
A diet study of flying lemurs by Wischusen (1990) showed that they do not
feed on leaves of dipterocarp trees. The trees they feed on are common in mid-
elevation to montane forests. Flying lemurs is not a threatened species as well.
Because the flying lemur population in Mindanao appears stable (Heaney et
al.,2006) and its diurnal roosting habits seem to make it more vulnerable to eagle
predation, the flying lemur will most likely remain as the prime prey item in
In studies of large tropical raptors like the Harpy Eagle and African
Crowned Eagle, prey remains collected on the nest and at the base of the nest
tree is a major source of diet information (Rettig, 1978; Galetti and Carvalho,
2000; Mitani et al.,2001). In contrast, in the case of the Philippine Eagle where
prey is dismembered, bones swallowed and the skull of large prey items
discarded elsewhere, collection of prey remains on the nest and under nest trees
to study diet is not appropriate. Direct observation of nest and probably video
recordings from hidden nest cameras that are installed way before the egg laying
period appear to be the best means to study and quantify diet.
2. Philippine Eagle hunting techniques
The wide array of prey items that resulted from studies by Gonzales
(1968), Kennedy (1985) and this study highlights the diversity of hunting styles
employed by the Philippine Eagle to catch prey. Predation on nocturnal
mammals seems to entail aptitude at tracking roost sites or tree cavities from
which to pick off resting animals. The relatively slender legs of the Philippine
Eagles have been speculated to be an adaption to taking out mammals inside
tree cavities (Kennedy, 1981). For macaques, deer, hornbill and snakes, slyness,
speed and an element of surprise appear crucial. Crowned Hawk-eagles (Mitani
et al.,2001) sit and wait in the canopy or swoop down from high to take on
monkeys. Philippine Eagles appear to employ the same method. Kennedy (1977)
documented direct pursuit of prey as a hunting strategy, which seems important
for hunting along valleys where visual track of prey animals can be maintained.
3. Predation on domestic animals
Domestic animals as prey items were recorded at the Apo nest. The
domestic chicken Gallus gallus bankiva was relatively the most represented
(n=6), and was almost exclusively brought in by the female (n=5). The male was
solely responsible for the two records of domestic cats Felis domesticus on the
nest, but contributed only one of three puppies Canis familiaris brought to the
nest. Together, domestic prey represented 16% of the total identifiable prey at
the Apo nest. In the pooled data, however, domestic animals represented only a
mere 7.3% of prey items, and contributed only 3.0% of the total prey biomass.
These observations represent the first confirmed record of domestic prey brought
to the nest by Philippine Eagles.
The first published report of domestic animal predation by Philippine
Eagles was perhaps by Ogilvie-Grant in 1897 when he wrote that it “…visits the
villages and carries off domestic poultry” and Whitehead in 1899 when he said
that the Philippine eagle is “well known to the natives as a robber of their poultry
and small pigs” (in Collar et al., 1999). Gonzales (1968) commented on domestic
predation at a nest in Davao del Sur: “a native woman…asked me to reimburse
her piglet, allegedly taken by one of the parent eagles”. The study of the Apo
nest provided the first evidence for domestic animal predation in Philippine
eagles, with cats, puppies and domestic chicken altogether representing 16% of
the total prey items. No domestic animals were ever brought to the four other
nests, nor were there one in the single nest studied by Gonzales (1968) or the
five nests studied by Kennedy (1985). Upland residents consistently report heavy
predation on domestic animals, and often justify shooting or trapping of Philippine
Eagles as a way of alleviating their losses. Although we got evidence from a
single nest, it is important to look further into the occurrence of domestic
predation especially since Philippine Eagles (as will be discussed further later)
has the ability to shift its preference of prey to whatever is available and easier to
catch and handle.
Philippine Eagles do take domestic prey, and an interesting challenge that
comes out from this result is how to convince the public not to persecute the
birds for taking their livestock and pets. Denying such knowledge to the public is
out of the question, as this would certainly cause more harm than benefits. From
the viewpoint of a resident who has personally experienced livestock or pet loss
from the eagles for example, continued denial can lead to feeling of animosities
not only to the bird but also to conservation practitioners. It can potentially breed
more hate than care and sympathy for the bird and the cause to conserve it. In
the uplands where opinions are easily taken for facts, providing the right
information and transparency is critical.
An effective strategy to address people and eagle conflict as a result of
domestic predation can be drawn from the nest site at Apo. Reports of domestic
predation were common long before this study. In 1996, a community-based
conservation (CBC) program was launched by PEF which included setting up
and strengthening a local cooperative, training and material assistance for
sustainable upland agriculture and marketing, soft financial loans, and provision
of a marketing vehicle. That project was fully turned over to the community in
2002, and the pair has continually bred since then in this disturbed nesting site,
with another young successfully fledged in March 2008. Despite the occasional
domestic predation which probably happens with each nesting cycle, five
successive nesting attempts were recorded since 2003, and resulted in the
successful fledging of 3 young (60% breeding success). As a result of investing
in CBC at Apo, the community maintains a positive attitude towards the eagles
and has considered the occasional loss of livestock as their generous
contribution to an imperiled eagle neighbor. CBC approaches are a tested tool for
effective species and habitat conservation (Horwick and Lyon, 2007), and should
be implemented in each nesting territory where human-eagle conflicts exist.
4. Philippine Eagle diet and food habits at Apo and Kitanglad nests
Substantial prey information was recorded from the Kitanglad and Apo
nests where a lot of observation efforts were spent. Interestingly, the nest
differed in terms of prey diversity. Prey items at the Kitanglad nest were all
mammals, whereas reptiles and birds formed part of the diet at Apo in addition to
mammals. The pair at Kitanglad had a narrower Food Niche Breadth (FNB, 3.93)
than at Apo (8.31). However, the estimated Mean Weight of Prey (MWP)
delivered was heavier at the Kitanglad nest than at the Apo nest. The average
prey biomass per day delivered at the Kitanglad nest is heavier and consisted
mostly of medium to large mammals that accounted for a heftier biomass (Table
9). On the other hand, the prey brought by adults to the Apo nest included lighter
reptiles and birds. Dietary overlap between the two pairs was 0.61.
Food-niche breadth (FNB), dietary overlap, and estimated mean weights (g) of
prey of Philippine Eagle pairs at Kitanglad and Apo during the nesting season in
Total prey individuals 52 73
Mammal total individuals 52 60
Bird total individuals 7
Reptile total individuals 6
Mean weight of prey (g) 1758.94 1107.23
FNB 3.93 8.31
Dietary overlap 0.61
The difference in the prey composition in the diet between the Kitanglad
and Apo eagle pairs illustrates the breadth of prey that the eagle can take. An
overlap index of 0.6 indicates diet similarity, but also a certain level of difference
that is worth looking at. Six taxa were unique to the Apo nests, namely bats,
rodents, snakes, birds, domestic cat and domestic dog. Based on mean prey
weight calculations, the Kitanglad pair went for heavier prey items. Contrasting
the habitat types at these two sites, one can not help but ask whether such
differences in prey taken were a product of habitat quality in these two sites. For
example, the Apo nest site is very much disturbed whereas the Kitanglad gets
very little human interference. Kitanglad has a greater mammalian diversity
(Heaney et al., 2006) and has better protection and management measures than
Apo. The fact that the pair at Kitanglad specialized on a few native and heavier
mammalian prey while the pair at Apo used a range of vertebrate groups,
including domestic animals, points to the possible relationship between habitat
quality and prey preference in Philippine Eagle pairs. It is hypothesized that
eagle pairs in large and undisturbed habitats tend to specialize on moderate to
large arboreal mammals, whereas those in disturbed areas have a broader
spectrum of vertebrate prey, including animals associated with highly disturbed
areas. Corollary to this hypothesis, it also predicted that whenever heavier
arboreal mammals are in low densities, the pair would compensate by taking on
more of a wide variety of smaller prey items.
5. Size difference between prey items hunted by male and female eagles
The size of prey delivered by female and male eagles varied across the
post-brooding phase (Fig. 9), although pooled data indicated that the females
delivered more of the smaller prey (i.e. weighing less than 1 kg). In contrast,
males delivered more animals within the 1,000 to ≥4,000 g range. During the
brooding phase at the Kitanglad nest, the male exclusively brought prey items
within the 1,000 to ≥4,000 g range. These animals were the Philippine flying
lemur (n=2), Common palm civet (n=2), and Long-tailed macaque (n=2).
Calculating the total biomass of prey at the three nests, the male had a bigger
share (91,888 g). The sum of the females’ hunts (66,921 g) is only 72% of the
total biomass contribution of the males’.
Fig. 9. Comparison between the prey mass categories brought by male and
female during the post-breeding phase in three Philippine Eagle nests observed
(Cabuaya, Apo, Kitanglad) in Mindanao. 1= 1–499 g, etc
It has been hypothesized that the size difference among raptors where the
females are lager than the males (i.e. reversed sexual dimorphism) is an
adaption so each sex can exploit different prey species (Selander, 1969).
Another hypothesis suggests that smaller males make hunting of small and more
abundant prey more efficient, whereas larger females can take on larger prey to
help meet the food demands of a growing chick during the latter stage of nesting
(Reynolds, 1979). Looking at the biomass of prey brought to three nests, both
males and females hunted large and moderate-sized prey items. In fact, the male
brought in more of the heavier prey items on the average (e.g. macaques, civets
and flying lemurs) than the female, and the female seemed to have specialized
on smaller prey items than the male. At two nests, prey brought during the post-
brooding period was likewise variable, with males also bringing in heavier prey
than females. Thus, these results do not seem to support the prey augmentation
Sexual dimorphism among Philippine Eagles, whose diet is mainly
mammals, can be considered moderate. Calculating for its dimorphism index by
getting the ratio of female to male wing length, the value for Philippine Eagles is
about 1.3 (Appendix H). Raptors of the genus Falco and Accipiter exhibit
moderate to extreme dimorphism with an index ranging from 1.10–1.70
(Appendix I, data from Clark, 1999). According to Newton (1979), raptors that
feed on fast and agile prey, like birds, exhibit stronger dimorphism than raptors
that either exclusively feed on mammals or raptors that have a bird-mammal
combination in their diet. The extent of dimorphism among Philippine Eagles
seems to conform to this general trend in diet and dimorphism.
6. Breeding failures as an effect of food stress
This study showed two possible ways by which low food delivery rates can
lead to breeding failures: one is egg desertion and the other is nestling desertion.
In both cases, the egg/chick becomes vulnerable to predators and the elements
in the absence of the attending female. The study also demonstrated that prey
biomass rather than the number of prey individuals becomes more important in
assessing how much nourishment is necessary to ensure successful
reproduction. The abundance and availability of prey is critical to the breeding
and survival of the Philippine Eagle. In fact, several investigators showed that the
patterns of habitat use by raptors are very much dependent on the distribution of
prey within such habitat (Newton, 1979; Ward et al.,1998). However, the
influence of prey density and availability on Philippine Eagle ecology, behavior
and productivity has not been given the rightful research attention it deserves as
far as studying prey base is concerned.
G. Estimates for Philippine Eagle adult longevity
Using Brown’s (1966) formula, it is estimated that in a stable population an
adult Philippine Eagle must live 29-30 years on the average before it can breed a
young that will reach sexual maturity (Appendix G). This age is not impossible
because a captive female at the Philippine Eagle Center is 46 years old now and
still alive. However, with the occurrence of human-induced mortalities which
might already be causing recent instability in the eagle population, the calculated
longevity estimate might represent a minimum value only.
Preventing high death rates among breeding adults, therefore, is very
critical in the light of the estimated longevity of adult Philippine Eagles. If many
adults die before reaching the average adult longevity, few young eagles will be
produced to replace dying adults. Coupled with high juvenile mortality, continuing
damage to nest sites, declining prey base, deforestation, and widespread apathy
of Filipinos to the plight of the species, the eagles will inevitably fall into the
vortex of extinction.
SUMMARY, CONCLUSION AND RECOMMENDATIONS
This thesis described the initial results of a continuing effort to document
the nest and nest tree characteristics and the breeding success, biology,
behavior and food habits of nesting Philippine Eagle pairs studied in Mindanao
from 1999 to 2007. Baseline values for breeding behavior parameters, (i.e., nest
attendance, incubation and brooding bout duration, trip duration and feeding
rates during each stage of breeding) were provided. The preliminary estimate of
longevity illustrated the critical value of ensuring the long-term survival of
breeding adults to maintain population stability.
Results for breeding behavior and food habits were not very different from
previous studies, but this study provided details and additional insights. Flying
lemur (Cynocaphalus volans) is the primary prey species, and arboreal mammals
as the most important prey group. Because arboreal mammals appear to exist in
stable numbers in mid-elevation forests that are not immediately under human
threat, these animals will most likely remain as an important prey group in
Mindanao. Philippine Eagles can shift their diet to what is available. When
heavier, native mammals are scarce, they seem to adjust by taking on more of
smaller prey items. In contrast to the prediction that females in sexually
dimorphic raptors tend to take on large and heftier prey than males, the reverse
seems to be true for Philippine Eagles.
This study also provided the first clear evidence of predation on domestic
animals and pets by Philippine Eagles. Because they are opportunistic feeders
and because domestic predation may result to eagle mortalities if people shoot
the birds to alleviate their losses, predation by Philippine Eagles needs to be
investigated further. The study also provided insights into the status of nest sites
and breeding pairs and gaps in research information that needs to be addressed.
Because it is long-lived, has an average longevity of 29 to 30 years, and is
very dependent on a tropical forest that is fast-disappearing, the Philippine Eagle
is very vulnerable to extinction. With the loss of large trees in lowland forests,
many nest sites are concentrated along forest edges near upland communities,
making nesting adults vulnerable to human persecution. Being opportunistic
feeders, they are predisposed to capturing domestic prey when native prey is
limiting. Predation on domestic animals makes the eagles more susceptible to
shooting and trapping.
There is evidence that some pairs suffer from lack of food that may result
to breeding failures through egg or nestling desertion. But there is evidence too
that eagle pairs near large, relatively intact forests where stable populations of
arboreal mammals exist are breeding well. Prey abundance and availability is
important for the long-term survival of the Philippine Eagle.
Because of the importance of increasing adult survival as well as ensuring
productivity, protection of breeding Philippine Eagles and the places where they
nest are important. Conservation of the home range where they forage is equally
important to ensure that enough prey base is available. Community-based
conservation (CBC) approaches can be a potent tool for conservation in places
where people and Philippine Eagle conflicts are tense.
This study provided evidence that the Philippine Eagle has the longest
and energetically most expensive parental care for any birds of prey. Because of
its breeding biology and the fact that the species is long-lived, it is very
vulnerable to extinction.
This study supports a previous suggestion that breeding failures are not
the primary cause of decline. In the light of the trends in human persecution,
deforestation and the fact that the species is long-lived, adult mortalities seem to
be the major cause of decline.
Philippine Eagles in Mindanao nest exclusively on large trees, which could
already be limiting. Some Philippine Eagles have no choice but to nest at forests
edges near communities where most large trees are confined.
Patterns of courtship, aerial displays, nest-building, and copulation rates
are similar to those known for other birds-of-prey. Philippine Eagles invest on
frequent copulation perhaps to strengthen pair bond, and on sprig delivery all
through out the nesting season probably as a means to rid the nest of ants, flies
The flying lemur (Cynocephalus volans) remains to be the dominant prey
item, and arboreal forest mammals as the most important prey group in
Mindanao. Philippine Eagles are opportunistic feeders. Initial data suggests that
pairs in disturbed habitats feed on more prey species that weighed less whereas
those in intact, less disturbed forests feed on fewer but relatively heavier prey.
Philippine Eagles feed on domestic animals, but this maybe happening
only in heavily disturbed nesting territories where natural prey items are scarce.
Predation of Philippine Eagles on domestic animals and pets has conservation
implications as it makes the birds even more susceptible to hunting or shooting.
Each breeding adult must live for an average of 29 to 30 years so it can
produce a young that would replace it. Conservation strategies that increase
adult survival are important to population stability.
1. Information from birds nesting in forest interiors must be collected. The
information provided in this study is limited to observations of pairs in forest
edges, and may not accurately represent the Philippine Eagle population as
a whole. There could be eagles nesting at the north and north-east end of
Mindanao, particularly at Mt Hilong-hilong, Pantaron Range, and the forests
of Lanao Provinces, where huge tracts of lowland forest are still found.
2. More ecological information is needed from pairs on other islands (i.e.
Luzon, Leyte, and Samar islands) to determine general patterns and as well
as uniqueness in island populations.
3. Comparative studies that look at behavior, productivity and survival between
edge-nesting and interior-nesting eagles, as well as between birds in small
patches against those in larger forests must be conducted.
4. More prey base studies are needed, including assessing the relationship of
prey abundance and availability to home range size, movement patterns
and breeding success.
5. Radio and remote telemetry techniques must be employed to measure
habitat use, movement and dispersal of adults and dispersing young.
6. To complete our understanding of Philippine Eagle breeding biology and
ecology, studies of the post-fledging stage must also be done.
7. A study of nest site selection and GIS mapping and modeling of actual and
potential nest site must be done to know if nest sites are indeed limiting.
8. Adult and young eagles must be marked and banded as a means to know
mortality or turn-over rates.
9. Long-term monitoring of breeding in nest sites must be sustained.
10. For current conservation efforts, species-directed approaches much invest
on minimizing, if not preventing, adult mortalities.
11. For Philippine Eagles nesting in forest edges close to communities,
Community-based Conservation approaches must be implemented.
12. On-site education campaigns must be designed to prevent destruction of
nesting sites and shooting and trapping of adult eagles and their fledged