Breeding and Nesting Biology in Raptors

Abstract

Raptors are limited by suitable breeding habitat, and they have specific nest-site requirements. Habitats of high quality presumably have the resources required to sustain relatively high rates of survival and reproduction. High-quality individuals would occupy territories of higher quality and would have greater fitness. Many birds may use their own reproductive success to assess the quality of their territories, and breeding failure would act as a determinant for dispersal, increasing an individual’s propensity to move to a better habitat. Food supply, nest-site availability, weather conditions and bird experience seem to act through the body condition of the female and are known to limit raptor populations. Quality of nesting territories and breeding success vary widely with different factors.

Full text

63© Springer International Publishing AG, part of Springer Nature 2018
J. H. Sarasola et al. (eds.), Birds of Prey,
https://doi.org/10.1007/978-3-319-73745-4_3
Chapter 3
Breeding andNesting Biology in Raptors
LuisTapia andIñigoZuberogoitia
Introduction
Raptors are limited by suitable breeding habitat, and they have specic nest-site
requirements. Habitats of high quality presumably have the resources required to
sustain relatively high rates of survival and reproduction. High-quality individuals
would occupy territories of higher quality and would have greater tness. Many
birds may use their own reproductive success to assess the quality of their territo-
ries, and breeding failure would act as a determinant for dispersal, increasing an
individual’s propensity to move to a better habitat. Food supply, nest-site availabil-
ity, weather conditions and bird experience seem to act through the body condition
of the female and are known to limit raptor populations. Quality of nesting territo-
ries and breeding success vary widely with different factors.
In this chapter, we review the scope and objectives of breeding and nesting
biology studies in birds of prey updating scientic and conservation knowledge in
this eld.
Breeding andNesting Habitat Selection
Habitat selection is a consequence of natural selection that favours individuals that
preferentially settle in the best habitats, so that they maximize their biological effec-
tiveness (Fuller 2012). Therefore, the habitat selection process is related to habitat
quality and long-term population persistence through reproductive success
L. Tapia (*)
Department of Zoology Genetics and Physical Anthropology, Universidade de Santiago de
Compostela, Santiago de Compostela, Galicia, Spain
e-mail: luis.tapia@usc.es
I. Zuberogoitia
Estudios Medioambientales Icarus S.L.Icarus S.L.C/San Vicente, Bilbao, Bizkaia, Spain

64
(Boulinier et al. 2008). In the case of raptors, breeding habitat is an essential
resource because it must guarantee food and protection for a long breeding period
(Lõhmus 2004; Tapia etal. 2007a). Thus, breeding-nesting habitat selection reports
on the basic requirements of raptors in a critical period of their life cycle (Orians
and Wittenberger 1991). Management and improvement of breeding raptors’ habitat
have broader positive effects on the whole of biodiversity, because raptor popula-
tions can act as valuable bioindicators of changes and stresses in ecosystems, as
they are sensitive to changes in land use, habitat structure and habitat fragmentation
and are highly susceptible to local extinctions (Sergio etal. 2005; Bednarz 2007;
Burgas etal. 2014; Donázar etal. 2016).
Raptors are limited by suitable breeding habitat, and they have specic nest-site
requirements (Newton 1979; Ferguson-Lees and Christie 2001). Each species has
its own preferences for the position, height, orientation, shelter, accessibility and
visibility of the nesting-site. The nest-site selections include upper parts of trees,
caves, cliffs and on the ground (see Tapia etal. 2007a). A shortage of nest sites may
also limit the density of a species below the carrying capacity of the habitat, for
instance, in cliff-nesting raptors or mature forest-dwelling species (Newton 1998).
Some species present a plasticity in nest-site selection, depending on geographic
area, habitat or intra- and interspecic densities. Some nest sites are repeatedly
occupied, even though the owners may change. In some cases, decades after becom-
ing extinct, when a species returns to an area, they reoccupy the very same nest sites
used in the past (Del Hoyo etal. 1994; Hardey etal. 2009).
Many species use old nests, and this strategy can save a good deal of time and
energy at the beginning of the breeding season. Resident breeding raptors often
have several nests in their territories and use them for breeding alternatively from
oneyear to another (Millsap etal. 2015; Slater etal. 2017). They may also use alter-
native nest sites for roosting or preparing prey items. Therefore, they are biologi-
cally signicant and warrant greater conservation consideration (Del Hoyo etal.
1994; Millsap etal. 2015). Management decisions should be based on alternative
nest-use patterns within territory (Slater etal. 2017).
Many ecologically relevant aspects related to the mechanism of habitat selec-
tion at different spatial scales are involved in the occupancy process of a high-
quality territory (Wiens etal. 1987; Newton 1998; Sánchez-Zapata and Calvo
1999). Study designs must be consistent with the abilities of the subject species to
perceive and move among existing habitat patches, and researchers should con-
sider the various scales at which habitat features may have inuenced (Litvaitis
etal. 1994; Morrison etal. 1998).
Breeding raptor habitat requirements sometimes are linked to the distribution of
prey. Because it is difcult to observe predatory behaviour in most raptor species,
the inuence of prey on habitat use by raptors often is inferred by comparing, at the
scale of activity areas, measures of prey abundance and raptor use among catego-
ries of vegetation types, structure or land uses (Graham and Redpath 1995; Marzluff
etal. 1997; Selas 1997a, b; Bakaloudis etal. 1998; Ontiveros etal. 2005; Marti
etal. 2007; Rodríguez etal. 2014). Use of land or vegetation structure by raptors
often is positively related to prey abundance (Selas and Steel 1998; Ontiveros etal.
L. Tapia and I. Zuberogoitia

65
2005; Rodríguez etal. 2014), but these relationships can be confounded by density
of vegetation. Predation is sometimes more intense in areas where vegetation is
sparser and less dense, regardless of prey abundance (Bechard 1982; Thirgood etal.
2003; Ontiveros etal. 2005; Rodríguez etal. 2014; Martínez etal. 2014), illustrat-
ing the need to distinguish, when possible, between prey abundance and availability
(Tapia etal. 2007a).
Raptors are typically selective with regard to breeding and hunting habitat (Janes
1985). At least three levels of spatial scale used by raptors during the breeding sea-
son may be considered– nesting area, post-edging family area (PFA) and foraging
area (see Tapia etal. 2007a). The nesting area is the area immediately surrounding
the nesting substrate, often contains alternative nests and may be reused in consecu-
tive years, and these nests should be given the same protection as used nest in land-
use planning (Millsap etal. 2015; Slater etal. 2017). The PFA surrounds the nesting
area and is dened as the area used by the family group from the time the young
edge until they are no longer dependent on the adults for food. The foraging area
is the area used by the provisioning adults and typically encompasses the remainder
of the home range during the breeding season (Fig.3.1). The inuence of habitat
features at different spatial scales is likely species-specic and can change with
body size, mobility and life history requirements.
Raptors typically range over large territories that include heterogeneous habitats
and landscapes (e.g. Pedrini and Sergio 2002; Martínez etal. 2003; Sergio etal.
2003b; Kudo etal. 2005; Watson 2010; Bruggeman etal. 2014; Tapia etal. 2017).
Raptor populations are limited by the availability of breeding habitat at the
microscale level (Bevers and Flater 1999), and different studies have been
conducted to elucidate habitat use and preference of nest sites (e.g. Selas 1997a;
Martínez and Calvo 2000; Finn etal. 2002; Poirazidis et al. 2004; Squires and
Fig. 3.1 Diagram of spatial scales used by raptors during the breeding season. (From Squires and
Kennedy 2006)
3 Breeding and Nesting Biology in Raptors

66
Kennedy 2006). However, studies of habitat associated with nesting activities that
occur at larger spatial scales such as the post-edging areas, foraging areas and
areas used during natal dispersal are less common (e.g. Daw and DeStefano 2001;
Bosakowski and Speiser 1994; Sergio etal. 2003a, b; Tapia etal. 2008a, b; Mañosa
etal. 1998; Balbontín 2005). Macrohabitat characteristics (vegetation cover types,
topography, human pressure, availability and accessibility of prey, etc.) of breeding
home range are also important components in nesting habitat selection (Janes
1985; Bosakowski and Speiser 1994; McGrady etal. 2002; Sergio etal. 2004;
Kudo etal. 2005; Rodríguez-Lado and Tapia 2012).
Modelling the distribution of breeding raptors has become more common in the
last two decades, given their utility as raptor conservation management tools (e.g.
Sánchez-Zapata and Calvo 1999; Sergio etal. 2003a, b; Bustamante and Seoane
2004; Tapia etal. 2007b, 2017). The rapid development of geographic information
systems (GIS), and more recently the use of satellite earth observation data in com-
bination with habitat suitability modelling techniques, has facilitated the handling
and management of environmental data at increasingly larger spatial scales and pro-
vides repeatable, standardized and veriable information for long-term monitoring
of environmental indicators (Corsi etal. 2000; Sarasola etal. 2008; Franklin and
Miller 2009; Pettorelli etal. 2014; Mairota etal. 2015). This type of approach, using
raptors as biodiversity surrogates, is particularly suited to highly dynamic land-
scapes where the spatial interactions between the different ecological drivers
strongly determine ecosystem functioning (Sarasola etal. 2008; Rodríguez-Lado
and Tapia 2012; Gonçalves etal. 2015; Regos etal. 2017; Tapia etal. 2017).
Studies of breeding raptors’ habitat descriptions in management plans should
account for temporal changes in habitat preferences. A relatively long time scale
would be an examination of the effects of land use and disturbances in dynamic
landscapes on the raptors’ habitat and protected areas’ monitoring and conservation
(Regos etal. 2017; Tapia etal. 2017). Some resident raptors use particular habitats
during specic periods of the year, and only an assessment of habitat use during the
full annual cycle would describe the species habitat preferences (Tapia etal. 2007a,
2008a; Hardey etal. 2009).
Several studies have identied apparent discrepancies between identied habitat
preferences and breeding success for different raptor species (see Chalfoun and
Schmidt 2012). This ecological effect is often discussed in the context of Ecological
Traps (suboptimal habitats for breeding or population growth but which attracts
individuals). Ecological traps have been observed particularly in environments
altered by man that particularly affect birds of prey (Cardador etal. 2015; Torres-
Orozco et al. 2016; García-Salgado 2017). Therefore, in order to know the real
quality of raptors breeding habitats, it is convenient to study the relationship between
habitat preferences and breeding success.
Limitation of breeding density by nest-site shortages is widespread among dif-
ferent species of raptors that use special breeding substrates, whether they are cliffs,
burrows or scarce trees in open land (Negro etal. 2007). Breeding raptors often use
articial structures to support their nests or nest in abandoned nests of other species.
For such species, nesting platforms can be used to increase nest availability, and this
L. Tapia and I. Zuberogoitia

67
management technique is particularly useful for increasing and maintaining stable
raptor populations. Installation of nest boxes can also provide nesting sites for some
raptor species. Populations increased faster in areas with nest boxes than in areas
without articial nests and may reduce the effect of rodent pests (Paz etal. 2013).
Territoriality andHabitat Carrying Capacity
One of the basic conditions for the reproduction of raptors is the existence of breeding
territories or hunting ranges (Newton 1979). The acceptable criteria for conrming
occupation territory vary between raptor species due to the differences in their breed-
ing behaviour and breeding habitat (Hardey etal. 2009). Territorial behaviour is a
fundamental spacing system that limits breeding densities of raptors (Newton 1979;
Brown 1989). This system informs the neighbours of the occupants of the territory and
prevents them from crossing its boundaries. In raptors, with a large ratio of nonbreed-
ers to breeders, it is usual for some individuals to wait several years beyond achieving
physiological maturity or adult plumage before they breed (Del Hoyo etal. 1994).
Spatial distribution pattern of territories in suitable habitat is usually more or less
regular, except in areas where human persecution is very intense. The natural rea-
sons for deviating from this regularity are the non-uniform distribution of resources,
especially food and nest-site availability (Newton 1979). Some raptor species may
nest isolated when resources are distributed uniformly and predictably, while they
do so in aggregate form when resources are unpredictable or concentrated in certain
areas (Solonen 1993; Del Hoyo etal. 1994). This pattern of distribution has also
been related to episodes of human disturbs and persecution (Newton 1998; Martínez-
Abraín etal. 2010) and by the attraction for co-specics (Donázar et al. 1993).
Interspecic territorial behaviour occurs between sympatric species with morpho-
logical similarity and similar trophic requirements and/or nest sites (López-López
etal. 2004; Rebollo etal. 2011; Rodríguez etal. 2017). There may be different types
of ecological interactions such as mutualism, predation, facilitation, etc. among
sympatric breeding raptors, which are essential to understand their spatial relation-
ship (Rebollo etal. 2011).
Territorial behaviour implies a “trade-off” between the energetic cost of search-
ing for and maintaining a territory and its potential benets (Gordon 1997; Adams
2001). It is commonly assumed, therefore, that the choice of a breeding nest site is
produced by natural selection (Martin 1998; Jones 2001), and the frequency of
occupancy is generally considered a measure of territory quality (e.g. McIntyre
2002; Sergio and Newton 2003; Rodríguez etal. 2016). In some species human
disturbances (e.g. timber harvest) in nest-site areas are associated with lower nest
site and territory occupancy. However, if pairs do nest at timber harvest sites, their
reproduction appears unaffected by these harvest activities, which would indicate
individual differences in tolerance to human disturbances and/or previous breeding
experience (Rodríguez etal. 2016).
3 Breeding andNesting Biology inBirds ofPrey

68
Site delity is generally inuenced by the breeding experience of individuals
(Serrano etal. 2001; Sergio and Newton 2003; Schmidt 2004). A positive correla-
tion between site delity and prior reproductive success (or, rather, between breed-
ing failure and territory change) is a common nding for breeding raptors (Martínez
etal. 2006; León-Ortega etal. 2017).
Information on the presence/absence of a raptor species in an area, although
potentially an indicator that the area constitutes habitat, tells little about its quality.
In contrast, measures of abundance of a given species in an area often are indicative
of the relative quality of the area as habitat although it may be misleading in some
situations (Van Horne 1983; Pulliam 1988). Perhaps the best indicators for assess-
ing habitat quality for a given species are estimates of productivity and survival or
combinations of both (e.g. rate of population change, λ). Unfortunately, these mea-
sures are difcult to obtain in short-term studies (see Tapia etal. 2007a). Estimating
survival is especially problematic in small populations (Beeisinger and McCullough
2002; Steenhof and Newton 2007) and usually requires monitoring marked birds
(e.g. banded, VHF telemetry, GPS satellite telemetry, stable isotopes) over extended
periods and large spatial scales (see Bird and Bildstein 2007).
Habitats of high quality presumably have the resources required to sustain rela-
tively high rates of survival and reproduction. Directly measuring the required
resources present in an area (e.g. availability of prey or nest sites) is one way to
assess habitat quality, but it requires that resources needed by the species in question
are known and that resources measured are available for use (Marti etal. 2007).
Another approach for assessing habitat quality is based on indicators of population
health. Variation in territory or habitat quality “habitat carrying capacity” can act in
the same way, reducing the average breeding rate in the population as numbers rise.
The concept of carrying capacity encapsulates the notion that, in any area of habitat,
resources must ultimately limit the numbers of raptors that can live there. But car-
rying capacity depends not only on features of the habitat, such as cover, food or
nest sites, but also on raptor behaviour. Another concept of carrying capacity, which
takes into account resource levels, concerns the “sustainable population size”,
which is the maximum number of individuals that can use a site over a dened
period (see Newton 1998; Beeisinger and McCullough 2002).
While some raptor species live in the same areas and territories year-round, oth-
ers become widely distributed between the breeding and nonbreeding seasons, and
yet others spend the breeding and nonbreeding seasons in geographically separate
areas. In addition, many species eat different types of food in the breeding and non-
breeding seasons, which may differ in their availability. For these two reasons, the
total carrying capacities of the breeding and nonbreeding habitats of particular pop-
ulations do not necessarily have to correspond, so that, as raptors return to nesting
areas each year, they compete for territories or nest sites (Del Hoyo etal. 1994).
High-quality individuals (e.g. the more experienced) would occupy territories of
higher quality (leading to higher occupancy rates for those territories) and would
have greater tness (Calsbeek and Sinervo 2002; Rutz etal. 2006; Martínez etal.
2006). Many birds may use their own reproductive success to assess the quality of
their territories, and breeding failure would act as a determinant for dispersal,
L. Tapia and I. Zuberogoitia

69
increasing an individual’s propensity to move to a better habitat (Hoover 2003).
In fact, for some species, site delity is conditioned by variations in habitat quality,
and high-quality sites may outweigh the inuence of previous breeding experience
(Bried and Jouventin 1999; Jiménez-Franco etal. 2013).
Many studies have demonstrated that competitive interactions, both intra- and
interspecic, play an important role in the spatial distribution and breeding habitat
selection of raptors (e.g. Sergio etal. 2004; Martínez etal. 2008; Rodríguez etal.
2017). Territorial breeding raptors show a negative relationship between density and
average productivity (Sergio and Newton 2003), as well as an increase in the pro-
portion of low-quality territories occupied as the population size increases (Ferrer
and Donázar 1996; Newton 1998; Krüger and Lindström 2001a).
Natal Dispersal andFloaters: Ecological Importance
Populations of raptors are composed by a sector of territorial breeders and a sector
of nonbreeding individuals, “oaters”. These individuals may be located far away
from the breeding grounds or closely coexisting with territorial holders (e.g. see
Prommer etal. 2012; Tanferna etal. 2013; Zuberogoitia etal. 2013b). Survival of a
raptor population is strongly dependent on the dynamics of “oaters” and on the
number of available settlement areas (Penteriani and Delgado 2009).
A substantial oating population of nonbreeding birds remains within territories.
Nowadays it is not possible to determine the status of oater populations of most
raptor species because nonbreeding birds are difcult to study due to their cryptic
behaviour, differential habitat selection, spatial separation from breeders or poten-
tial long-distance dispersal (Tanferna etal. 2013). In some species of raptors, non-
breeding individuals may represent more than 50% of the total population (Kenward
2006). In most raptor species, nonterritorial birds are generally younger than territo-
rial breeders but often show a marked hierarchy, with the more dominant ones
acquiring territories before the others.
Floaters normally wait for opportunities to occupy a breeding territory which in
turn are conditioned by the health of the breeding populations. The presence of
nonbreeders is difcult to detect but repeatedly conrmed by the observations of
rapid replacement of lost mates (by human persecution, electrocution, collision
fatalities, etc.) in several raptor populations (golden eagle Aquila chrysaetos, north-
ern goshawk Accipiter gentilis, Eurasian buzzards Buteo buteo, etc.) (Newton 1979;
Kenward 2006; Watson 2010; Gil-Carrera etal. 2016). But in stable populations
with low adult turnover rate, oaters may spend several years waiting for a vacancy.
In fact, the time lapse between the loss of a breeder and the establishment of a new
individual, the average age at rst breeding and the occupancy rate of territories are
useful factors to measure the health of the oater population (Zuberogoitia etal.
2009).
Floaters can also negatively interact with those breeding raptor communities that
overlap in space (i.e. intraguild predation between dispersing individuals and their
3 Breeding andNesting Biology inBirds ofPrey

70
intraguild prey). Floating individuals may settle close to breeding pairs; thus, the
effects of the intraguild predator on its intraguild prey may be underestimated
because researchers often have not monitored the diet of these “invisible” oater
individuals (Penteriani and Delgado 2009).
Floaters may prefer to settle in habitats similar to their natal habitats, because (a)
this behaviour reduces the costs of assessing suitable new habitats, or (b) experience
in natal habitat improves performance if a raptor settles in the same habitat type
after dispersing (Stamps 2001). During dispersal, raptors are not territorial and may
homogeneously distribute themselves in space; however, dispersal patterns may
reveal that the distribution of individuals may be under constraints other than terri-
toriality or location of food resources (Mañosa etal. 1998; Penteriani and Delgado
2009, in this book Chap. 4).
Potential future breeders of populations may spend a large part of their lives in
high-risk areas. In fact, stochastic events, such as human persecution or collisions
with power lines or vehicles, can seriously increase mortality rates in temporary
settlement areas. Because conservation efforts for endangered raptor species popu-
lations focus primarily on breeding areas, conservation programs conducted in
breeding territories can be ineffective if the genuine problem is in the settlement
areas, and dispersing individuals may reveal the locations of crucial areas of conser-
vation interest (Ferrer and Harte 1997; Ferrer etal. 2015).
Quality ofNesting Territories
Food Supply andBreeding Densities
Food supply, nest-site availability, weather conditions and bird experience seem to
act through the body condition of the female and are known to limit raptor popula-
tions (Newton 1998; Mañosa etal. 1998; Dewey and Kennedy 2001). Clutch size
and breeding success increase markedly with an improved food supply, and food
shortage may reduce the population size through lowering breeding rates. Such posi-
tive relationships involve both numerical and functional responses to population
increases in their main prey species (Newton 1998; León-Ortega etal. 2017). This
effect may be hard to detect because of time lag between the food shortage and the
resultant decline in breeding numbers. In long-lived species as raptors, it may take
several years before the effects of poor breeding are reected in poor recruitment
(Watson 2010).
Raptors that have stable food supplies show the higher stability in breeding pop-
ulations recorded in birds. This level of stability has been recorded in a wide range
of species. Moreover, the same species may uctuate numerically in one region, but
not in another, depending on the stability of the local prey supply (Newton 1979,
1998). Raptors need regular trophic resources through the breeding season in order
to establish breeding territories. If trophic resources keep at high enough level for
feeding adults, they maintain territories throughout the year. This is the normal rule
L. Tapia and I. Zuberogoitia

71
for raptors living at low and medium latitudes. However, in some regions, mainly
northern distribution areas, raptors depend on the seasonal presence of resources
(migrant birds) or seasonal availability of some key prey species (micromammals,
sh), being obligated to migrate or develop a vagrant strategy.
Breeding success varies widely with geographical area, latitude, habitat quality
and altitude, but food supply determines a species breeding success at different
levels: the proportion of pairs actually breeding, the age of rst breeding, clutch size
and the quality of the eggs and thus, hatching success and the growth and survival
rates of nestlings and edglings (Del Hoyo etal. 1994). In addition, the availability
of the main prey types inuences oaters dispersal patterns and movements among
the different settlement areas. A reduction in food availability may increase oaters’
mortality rates within settlement areas, affecting the stability of breeding popula-
tions (Penteriani etal. 2006a, b).
Nest-Site Resources
Nest-site availability has been described as an important limiting factor for some
raptor populations (Franco etal. 2005). There are several examples of areas full of
favourable nest sites for certain raptor species but lacking regular availability of
trophic resources in which these species are absent or keep low densities. On the
opposite, other areas plenty of food but with low availability of nest sites show high
density of some species but can merge as sink areas due to the impossibility of them
to breed (e.g. see Fasciolo etal. 2016).
Most of the Accipitridae species are able to build their own nest with branches,
wool of ungulates and even plastics and rubbish (Fig.3.2). Thus although building
a new platform requires energy and time, most of forest raptors, rocky eagles and
vultures have several nests shared over their territories (Kochert and Steenhof 2012;
Jiménez-Franco etal. 2014). However, most Falconidae species, particularly those
of the genera Falco, do not build their nest and simply dig a bowl in the sand or
gravel of a ledge, pothole or cave of a cliff, building a hole in a tree or reuse plat-
forms of other species (e.g. Del Hoyo etal. 1994; Espie etal. 2004; Olsen 2014).
The number of eyries per territory depends on the availability of adequate rocky
substrate (cliffs, Zuberogoitia etal. 2015). In certain cases, one eyrie has been con-
tinuously used during decades, even centuries (Burnham etal. 2009).
Anthropic Pressure
Both trophic and nest-site availability may be seriously affected by anthropic
pressure. Most times this pressure has a negative effect because it produces habi-
tat loss, disturbances or direct persecution (Zuberogoitia etal. 2014; Donázar
3 Breeding andNesting Biology inBirds ofPrey

72
etal. 2016; Martínez etal. 2016, in this book Chap. 9), which cause both direct
and indirect reductions on breeding raptor populations. Most of the threats are
related to urban and suburban areas of all the continents, although new hazards
are also affecting pristine areas, pushing some raptor species to critical conserva-
tion status (Donald etal. 2013). However, anthropic pressure can reach wider
ranges because of the cascade effects of certain activities. For example, bearded
vultures and sea eagles were exterminated from most of their European distribu-
tion due to direct persecution, although now they are recovering again after
expensive reintroduction programs (Helander etal. 2008; Margalida etal. 2008a).
Although humans should learn from their errors, similar threat (the use of diclof-
enac, a veterinary drug) has recently caused the depletion of vulture populations
in Asia and Africa and poses a serious threat for European vultures (Ogada etal.
2015; Buechley and Sekerciogly 2016, in this book Chap. 19). Moreover, prod-
ucts of anthropic origin are contaminating most of the ecosystems, being con-
sumed by all the species, being raptors deeply affected due to their top position
in the trophic pyramid and bioaccumulation processes (Zuberogoitia etal. 2006,
in this book Chaps. 10 and 11).
Also different forms of outdoor recreation have different spatiotemporal activity
patterns that may have interactive or cumulative effects on raptor breeding biology
and conservation, through human disturbance, physical habitat change or both
(Spaul and Heath 2016). However, anthropic actions can turn a habitat previously
unsuitable to suitable for a raptor species (e.g. open elds turned into timber planta-
tions may favour forest raptors, Zuberogoitia and Martínez 2011). Novel habitats, as
urban areas, also originate opportunities to some adaptable species which nd cities
Fig. 3.2 Female common buzzard (Buteo buteo) feeding her two nestlings. Forest-dwelling
raptors build their own platforms, in this case in a pine tree. (Photo credit: Iñigo Zuberogoitia)
L. Tapia and I. Zuberogoitia

73
plenty of food, although the nal consequences for their populations are still unclear
(Donázar etal. 2016, in this book Chap. 8).
Intraspecic Interactions. Ideal Free Distribution andDespotic
Distribution
The ideal free distribution (IFD) assumes that there are no competitive asymmetries
among individuals and that all individuals are equally “free” to occupy any space in
the habitat. Thus, in environments where resources have a patchy distribution, rela-
tively high-quality areas are expected to contain more individuals than relatively
low-quality areas such that all individuals gain equal access to resources (Calsbeek
and Sinervo 2002). However, ideal free distribution is rarely detected in raptors, and
dominant individuals occupy the best areas and force less able competitors to unfa-
vourable areas. This behaviour is dened as ideal despotic distribution (IDD). The
higher-quality individuals would occupy the best areas, and this, in turn, would
result in higher occupancy rates and higher breeding success and would have greater
tness. Under the IDD the best territories should be the most frequently occupied
(Sergio and Newton 2003). In addition, because the better territories are occupied
rst, it is also generally assumed that their owners begin the reproduction sooner,
which usually has a positive effect on breeding success (Espie etal. 2004; Pagan
etal. 2009; Freund etal. 2017). However, some raptor species do not follow the
IDD.Booted eagles (Hieraaetus pennatus) in south-eastern Spain followed a ran-
dom nesting distribution, and territory occupancy rate was not signicantly related
to reproductive parameters (Pagan etal. 2009). Authors suggest that the lack of
strong environmental variability could determine these results.
Individual Quality andBody Condition ofBreeders
Under an ideal free distribution, equal competitors select habitats to maximize their
individual tness; however, under ideal despotic distribution or more complex dis-
tribution models, individuals are unequal competitors, and resources or territories of
highest quality are monopolized by the strongest competitors (Johnson 2007).
Zabala and Zuberogoitia (2015) suggested that individuals entering into a breeding
population already represent a selection of the best individuals of the oating popu-
lation, and thus, a rst selection against poor quality individuals takes place before
they establish as territory holders.
Several studies report higher reproductive success in preferred habitat territories
(Newton 1991; Martin 1998; Sergio and Newton 2003), and long-lived raptors have
been reported to move from low-quality to high-quality places as they age, and they
acquire more experience and dominant status (Newton 1989, 1991). However, this
3 Breeding andNesting Biology inBirds ofPrey

74
rule is not always followed, and, for example, none of the peregrines monitored by
Zabala and Zuberogoitia (2014) in a 17-year study switched territory regardless of
some instances of consecutive reproductive failures, similar to the results reported
by Krüger and Lindström (2001b) in an 11-year study on common buzzards. Some
authors suggest that although site quality is a major determinant of tness, its effects
can be confounded with individual quality, a relationship that has been little studied
in large long-lived raptors (Carrete etal. 2008; Cardador etal. 2012). However, indi-
vidual quality can vary with age (Margalida etal. 2008b; Nussey etal. 2008), and
productivity can also be inuenced by trophic breadth and other denso-dependent
and denso-independent factors (Margalida etal. 2012; Newton 2013).
Zabala and Zuberogoitia (2014) also showed that there was an inter-gender dif-
ference in the individual effect, mainly due to the different roles of birds during the
reproductive period. For example, in most raptor species, males provide females
and offspring with alternate prey items when staple prey is in short availability and
experienced males can be better than younger ones in providing alternate prey
(Sasvari etal. 2000; Sasvari and Hegyi 2002; Katzner etal. 2005). Higher-quality
males seemed to deliver more prey in any circumstances regardless of territory
(Zabala and Zuberogoitia 2014; Pérez-Camacho etal. 2015).
Females need to reach a minimum body condition in order to start reproduction
and to produce eggs, which demand a high amount of energy. Reserves are accumu-
lated through the weeks or months before laying. Normally, females reach maxi-
mum body condition values during the months before laying. Harsh winter
conditions can result in poorer body conditions for individuals, less energy available
for egg production and a reduced clutch size (Korpimäki 1988; Steenhof etal. 1997;
Sasvari and Hehyi 2002).
Interspecic Interactions
The nature of interspecic interactions could be based on both the competition
for food resources or nest sites or intraguild predation (IGP) (Holt and Polis
1997). It has been largely proved that the inuence of intraspecic interactions
may determine the selection of breeding habitat in multispecies assemblages.
Katzner etal. (2003) suggested that the coexistence of four large eagles in north-
central Kazakhstan was primarily determined by intraspecic nest spacing and
that interspecic effects appeared to be secondary. Martínez etal. (2008) showed
that interspecic relationships within cliff-nesting raptor community follow a
general pattern of dominance related to body mass. In this sense, IGP plays a
crucial role in structuring raptor communities (Lourenco etal. 2011). There is a
clear separation between super-predator species (apex or top predators) and
meso-predator species. When apex predators are removed from a community,
other predators may subsequently respond functionally or numerically to this
change, a phenomenon known as meso-predator release (Soulé et al. 1988).
Conversely, when top predators enter communities from which they had been
L. Tapia and I. Zuberogoitia

75
absent, responses by lower predators may reect a meso-predator suppression
effect. These changes may strongly affect species distribution, density, ecology
and behaviour (Rebollo etal. 2011; Buchanan 2012). Changes in IGP relation-
ships may be related to natural processes (e.g. recolonized territories, Hakkarainen
etal. 2004) or also are may be masked behind habitat alterations which force
raptors to concentrate on favourable habitats. Even scientic activities may affect
IGP relationships (Zuberogoitia etal. 2012).
Predation
Although it is not easy to assess the effect of predation on population levels (Newton
1998, 2013), predation largely affects nest-site selection, breeding behaviour and
breeding success of raptors (e.g. see Newton 1998; Sergio etal. 2003a; Sergio and
Hiraldo 2008). The main predators of raptors (eggs, nestlings and adults) are large
owls, carnivores and corvids. Only the largest eagles and vultures are relatively free
from predation pressure, although their nests may be occasionally assaulted by car-
nivores. The nest-site selection of the rest of the species is widely conditioned by
predation pressure. Forest-dwelling raptors are more affected by carnivore preda-
tion than rocky raptors, mainly due to the climbing ability of forest carnivores such
as martens, genets or felids (García-Salgado 2017).
Breeding Cycle Phases
Pairing
In raptors, the division of breeding duties between sexes is more marked than in
other birds, and they thus have obligatory biparental care (Korpimäkki and
Hakkarainen 2012). In most raptors, one male and one female form a pair and raise
a brood together, and they thus typically are monogamous. However, in some cases
more than two individuals participate in raising the offspring from a single nest. The
involvement of breeding adult birds in parental investment is largely affected by
their social mating system: monogamy, polygyny and polyandry may imply differ-
ent tasks for breeding males and females (Clutton-Brock 1991).
Cooperative breeding is widespread within diurnal raptors, occurring in 22 of 76
genera (29%) and 42 of 304 species (14%, Kimball etal. 2003). The majority of
those species consist of groups in which extra birds are primarily adult males (poly-
andry). Polyandry can again be subdivided into sequential polyandry and cooperative
polyandry (Faaborg and Bednarz 1990; Tella 1993; González etal. 2006). Generally,
all males participate in copulations with the female; thus, any male in a group may
sire offspring and is potentially related to the offspring they assist in rearing. The
other pattern, polygyny, occurs regularly in a very small proportion species in which
3 Breeding andNesting Biology inBirds ofPrey

76
multiple females lay into one nest or lay in separate, widely distributed nests
(Kimball et al. 2003). Finally, there are polygynandrous (or communal) groups,
composed of multiple females and males, in which all group members may contrib-
ute genetically to the offspring produced by the group (Gil et al. 2017).
Courtship
Courtship starts early, even some months before egg laying, and displays consist in
mutual soaring, chasing ights by the male on its mate, apping ights with syn-
chronized movements and diving ights focused on the nesting area with a high
level of variations depending on the species (Del Hoyo etal. 1994, in this book
Chap. 2). Frequency and duration of mating games increases close to the laying
dates, and males start to gift quarries to females. Some species courtship and copu-
lation start 2 months before egg laying (Kenward 2006; Margalida and Bertran
2010; Watson 2010), although the frequency of copulates reach its maximum peak
close to the laying date. Females become quite lethargic some days before the depo-
sition of the rst eggs, reducing progressively the hunting activity until null values.
Simultaneously, males tend to increase feeding rates of females as well as copula-
tion rates (for further discussion on courtship and copulation behaviour in raptors,
see in this book Chap. 2).
Nest Selection
Habitat characteristics, of course, also inuence the decision for breeding in one
nest site, and it largely varies between each species’ requirements (e.g. Newton
2013). However, nest-site selection (Fig. 3.3) is also conditioned by social cues,
such as past reproductive success of conspecics (public information) or location of
the information producers (location cues: the presence of conspecics or heterospe-
cics, Danchin etal. 2004; Mateo-Tomás and Olea 2011; Jiménez-Franco etal.
2014). Moreover, nest-site selection is also conditioned by the knowledge of the
territory and individual decisions to prevent future parasitic infestation and to avoid
giving cues to predators (Zuberogoitia etal. 2015).
Laying andHatching Dates
Larger species tend to lay before smaller species, which helps them to complete
their long breeding cycles. This intraspecic divergence is also marked at a latitudi-
nal gradient. The southern populations (in the Northern Hemisphere) lay earlier
than those located northernmost (Kenward 2006; Zuberogoitia and Martínez 2015).
L. Tapia and I. Zuberogoitia

77
The photoperiod and weather are the main factors regulating this gradient. This, in
turn, has been adopted by different populations over generations.
Juvenile, rst breeding females usually lay later than adults (Biljsma 1993;
Kewnward 2006). Therefore, mean laying dates signicantly differ between those
populations with high proportion of juveniles and those whose age composition is
well distributed and tend to older distribution. Laying date is critical since in most
avian species, breeding performance decreases over the season with early birds
having more success and productivity than late conspecics (Verhulst and Nilsson
2008) and individuals born earlier in the season being generally more likely to
survive and recruit (Wiens etal. 2006; Brommer etal. 2014).
Raptors are among the slower layers in birds. Small species usually lay eggs at
the interval of 2days, medium-sized species at 2–3days and large ones at 3–6days
(Newton 1979; Margalida etal. 2004). The time lapse between eggs and the begin-
ning of incubation determine the hatching date of every chick and may condition
its survival.
Incubation
In most raptors, females perform most or all the incubation, brooding and feeding
of the nestlings, while males provide most or all of the food for the family (e.g.
Newton 1979, 1986; Cramp and Simmons 1980; Krüger 2005). Incubation
involves a transfer of heat between parent and embryo, in order to keep the egg
temperature between narrow tolerance limits that are close to the optimal
Fig. 3.3 Male Egyptian vulture (Neophron percnopterus) carrying material to the nest while
female is waiting for him in order to continue repairing the platform. (Photo credit: Iñigo
Zuberogoitia)
3 Breeding andNesting Biology inBirds ofPrey

78
development temperature. In most avian species, this transfer occurs through the
brood patch (Lea and Klandorf 2002). A brood patch normally develops in both
sexes in raptor species where incubation is biparental (i.e. both sexes share incu-
bation tasks, e.g. vultures, Wolter etal. 2013; Bassi etal. 2016). However, most
raptor species are uniparental, where the male does not incubate or contributes for
short periods only. In this case the brood patch is absent or poorly developed in
males (Newton 1979).
Incubation period (IP) varies between species and depends among other
factors on the initial egg mass (IEM), although some species show a divergence
from this relationship (e.g. Eurasian sparrowhawk IEM=23, IP=35; peregrine
falcon IEM=43, IP=31; common buzzard IEM=51, IP=36; Deeming, 2002).
Small falcons (kestrels) have shorter incubation periods (28 days) than do
Accipiter of the same weight (sparrowhawks); and Haliaeetus eagles have shorter
incubation periods (35–38 days) than do Aquila eagles of similar and lower
weights (42–45days; Newton 1979). Large vultures have the longer incubation
periods, from the smaller species (e.g. 31–40 days in Cathartes vultures,
39–45days Egyptian vultures (Neophron percnopterus) to the largest vultures
(e.g. 53–60days in both American condors or 54–58days in Himalayan vultures
Gyps himalayensis; Campbell 2016).
Clutch Size
The general trend is for larger species to produce the smaller clutches (Del Hoyo
etal. 1994). Large vultures only lay one egg, although the Egyptian vulture and the
bearded vulture (Gypaetus barbatus) are the only Old World vultures whose clutch
usually has two eggs (Donázar and Ceballos 1989; Campbell 2016). The maximum
number of eggs laid by those species with larger clutches is normally conditioned
by food supply. Small falcons that eat rodents tend to lay larger clutches than bird-
eating falcons in the same area, and these in turn have larger clutches than insect
eaters (Newton 1979). In some exceptional conditions in which some pairs enjoy
plenty of food circumstances, clutches with a record number of eggs are detected
(Altwegg etal. 2014).
Parental Care andNestling Development
During the rst days after hatching, nestlings are continuously attended by their
parents to protect them from weather and predation. In uniparental species, females
continue with the brooding process in order to heat offspring, and males relieve
them but less often than they usually do during egg incubation, although males of
some species do not contribute to this task at all (Dare 2015). The male investment
L. Tapia and I. Zuberogoitia

79
also seems to decrease during post-hatching period in some biparental species
(Bassi etal. 2016).
During the rst weeks of life, nestlings are fed regularly by their parents, and
they show a high growth rate of body mass (muscles and bones). The regular intake
of food and the maintenance of stable temperature, thanks to parent care, let nest-
lings grow adequately (Fig.3.4). However, when some of these factors fail, the
growth rate decreases and delayed development is detected.
The growth rate changes when feathers start to appear in the tail and wings.
Some days later, depending on the species, nestlings are able to thermoregulate.
The energy intake is redirected to feather growth at the expense of corporal growth.
All the energy intake is transformed in plumage and in lesser extent to nish the
corporal development. During this period, parental care decreases progressively.
Nestlings do not need to be incubated, although females still cover them in adverse
weather conditions (excessive sun exposure, low temperatures and heavy precipita-
tions). Females of most species remain close the nest site in order to defend the
offspring against intruders or predators, although at this stage female contribution to
hunting task increases in order to attend the demand of nestlings.
Fig. 3.4 Female peregrine falcon (Falco peregrinus) feeding her four nestlings in an old platform
of common raven (Corvus corax). (Photo credit: Iñigo Zuberogoitia)
3 Breeding andNesting Biology inBirds ofPrey

80
Weather Conditions
Those raptor populations living in environments with relatively stable weather con-
ditions suffer low or scarce losses on breeding productivity (Bosch etal. 2015).
However, when weather conditions show higher oscillations, an increasing effect on
breeding performance is detected. Persistent or heavy rain and low or high tempera-
tures between the egg-laying period and the rst weeks after hatching have adverse
effects on the reproduction of most raptors (e.g. see Kostrzewa and kostrzewa 1990;
Carrillo and González-Dávila 2010; Zuberogoitia etal. 2011; Mihoub etal. 2012;
Anctil etal. 2014; Touati etal. 2017). Rainfall during the rst half of chick-rearing
period is negatively correlated with breeding success (Fig.3.5). During this brood-
ing phase, nestlings do not thermoregulate; therefore, rain increases the risk of
hypothermia (Elkins 2004). A high amount of rain even if only on oneday can be as
negative as a number of continuous rainy days (Zuberogoitia etal. 2014). Moreover,
rain during the chick-rearing phase prevents adults from foraging, resulting in food
shortage for the young (Penteriani 1997; Sergio 2003; Mcdonald et al. 2004;
Lehikoinen etal. 2009).
Fig. 3.5 Peregrines do not build nests; they lay eggs on a depression of the ground or in a platform
of other species. In this case, the eyrie is placed on a ledge of a cliff, inside a bush (Ruscus aculea-
tus) that protects the nestlings against rain and low temperatures. The female has just fed the three
nestlings and is going to leave the nest in order to nish the quarry herself. (Photo credit: Iñigo
Zuberogoitia)
L. Tapia and I. Zuberogoitia

81
Multiple Brooding
Multiple-brooding occurs infrequently in raptors and is generally restricted to either
smaller species with shorter nesting periods in conditions of prolonged food abun-
dance whenever they occur (Newton 1979; Mendelsohn 1981; Curtis etal. 2005). It
is also observed in co-operative breeders where additional adults enhance nestling
provisioning efciency, saving both time and energy (Malan etal. 1997). The black
sparrowhawk (Accipiter melanoleucus) and the northern crested caracara (Caracara
cheriway) are the only two relatively large, monogamous raptor species in which
multiple brooding has been recorded with any frequency (Morrison 1998; Curtis
etal. 2005). Nevertheless, there are still many poorly known species, mainly in
tropical forests, which may also display this behaviour (e.g. grey goshawk Accipiter
novaehollandiae (Riddell 2013)).
Sibling Aggression: Cainism
Due to the above-mentioned asynchronous hatching of some raptors, in large eagles
and those vulture species that lay two eggs, the last bird to hatch has the biggest
disadvantage compared with its older siblings. In favourable years, the antagonist
behaviour between siblings remains at low level when parents are able to obtain
enough resources for feeding them (Watson 2010). However, some species like the
bearded vulture and some eagles (e.g. lesser spotted eagle, tawny eagle and
Verreaux’s eagles) are considered obligate cainists; in these species, aggression
between nest mates always results in a death (Newton 1979; Simmons 1988;
Margalida etal. 2004). In medium-sized raptors, sibling aggressions are scarce, only
occurring in periods of hunger (e.g. northern goshawks, Kenward 2006; common
buzzards, Dare 2015), and not in all species (e.g. peregrine falcon). While in small
species, such attacks do not occur, even when the young are starving (Newton 1979).
In these cases, adults equally feed all nestlings independently of their age and sex.
Fledging Phase
The edgling behaviour varies between species, depending on the sex, size, hunting
or ying behaviour, habitat requirements and nest-site selection. Small- and
medium-sized forest species start to move around the nest some days before their
plumages are totally developed. The edglings explore closest branches during the
rst days and make short ights to neighbouring trees, scrambling back to the nest
whenever a parent arrives with food. Males normally develop earlier than females,
and they start to move several days before their female siblings. This behaviour is
shared by cliff-nesting species when they breed in cliffs with wide ledges that let
edglings explore the vicinity areas. On the contrary, when the nest site is a narrow
ledge, pothole or little cave, edglings wait until they are able to y for jumping for
3 Breeding andNesting Biology inBirds ofPrey

82
the rst time. The same occurs in large raptors, whose edglings practice wing
movements in the nest platform in order to tone up the muscles. Most edglings are
able to y at rst attempt, although some of them fail in this rst ight and land
wherever they can. This causes a non-valued mortality rate, as sometimes they die
from starvation in the landing site or are predated by carnivores.
During the post-edgling stage, most edglings are noisy when parents approach
with food, although hungry broods scream almost continuously. This behaviour
makes it easy for predators to locate noisy young (Newton 1986).
Dispersion
Studies of natal dispersal in raptors have focused on four key developmental stages:
(1) a post-edging dependence period ending in emigration from the natal environ-
ment; (2) a long transitional phase (often termed “juvenile dispersal”, synonymous
with transience); (3) provisional settlement in temporary settlement areas, where
individuals establish more or less stable home ranges; and (4) settlement at a breed-
ing site (Ferrer 1993; Delgado and Penteriani 2008; Weston etal. 2013). There are
huge intra- and interspecies differences among the four phases and the nal result
of dispersion.
The start of natal dispersion varies between individuals, from some days after
edgling to some months later. Normally, young birds venture out of their natal
home ranges (pre-dispersal excursions) prior to their emigration at the start of natal
dispersal (Zuberogoitia etal. 2002; Kenward 2006; Weston etal. 2013; Dare 2015).
During the juvenile dispersal period, home ranges usually are much larger than
those obtained for territorial individuals (Margalida etal. 2016). Some migratory
species spent the rst years of life on wintering grounds until they mature (Alerstam
etal. 2006), although most of them follow the same migratory pattern of adults
(Prommer et al. 2012). However, juveniles show more tortuous routes, slower
speeds and more stopover days than adults (Mellone etal. 2013). Natal dispersal
distances tend to be lower for males than for females; in other words, males tend to
come back close to the natal grounds (Philopatric behaviour), while females usually
disperse longer (López-López etal. 2013; Newton 2013, for further information on
raptor dispersion, see in this book Chap. 4).
Sexual Maturity
Most raptors have one or more immature or subadult plumages before acquiring the
denitive adult dress (Newton 1979). Falconidae species and most small- and medium-
sized accipitrids moult all the plumage in oneyear, and therefore immature plumage
is changed for adult one during the second calendar year. However, some medium-
sized accipitrids need more than oneyear in order to complete one moult cycle, while
L. Tapia and I. Zuberogoitia

83
large raptors need several years (from 2 to 4years; Snyder etal. 1987; Zuberogoitia
etal. 2013c). In some species the immature plumage is changed rst for a subadult
plumage and later for an adult dress (Watson 2010; Zuberogoitia etal. 2016).
Bearded vultures reach denitive plumage in their 6th calendar year (Sesé 2011;
Zuberogoitia et al. 2016), approximately at the same age at which they start to
establish a territory (López-López etal. 2013). This, in turn, reects the importance
of the relationship between the acquisition of denitive plumage, the reach of sexual
maturity and the settlement in a breeding territory (Rohwer etal. 2011). However,
in the wild, some individuals begin breeding before the acquisition of the fully adult
plumage. Birds breed at a younger age than usual when conditions are especially
good, either in favourable areas or years, or when depleted population leave territo-
ries vacant (Newton 1979). This last case, in fact, has been suggested as reliable
warning signals of adult mortality or breeding performance (Balbontin etal. 2003;
Ferrer etal. 2003; Zuberogoitia etal. 2009). It is also common to nd subadults
breeding during the rst phases of recently restored populations (e.g. see Cade and
Durham 2003; Gil-Carrera etal. 2016).
Immature individuals show a lower breeding performance than adults (Ferrer
and Bisson 2003; Margalida etal. 2008a, b), possibly due to a delayed maturation
of the gonads. However, immature birds that enter the breeding population in satu-
rated populations are few and might be of higher quality than other individuals of
the same age (Zabala and Zuberogoitia 2014).
Management toIncrease Reproductive Success
Raptors are long-lived species, and thus adult survival is the demographic parameter
that contributes most to breeding success and population growth. Management tech-
niques aimed at increasing productivity:
Clutch manipulation vulnerable eggs can be removed after the start of incubation
and replaced with articial eggs, so that incubation continues. The real eggs are
incubated articially, and the young produced are returned to the nests. The overall
production of young obtained should be higher than if the original eggs had been
left with the pairs. This method has been used successfully with different raptors
(Olsen and Tucker 2003).
Brood manipulation the number of young reared to independence can be aug-
mented by increasing brood size to the normal maximum for a species. This has
been done with species that experience death of young nestlings due to fratricide
(siblicide or cainism). Brood size is reduced to one by removing nestlings at an age
before sibling rivalry develops. These young are hand-reared and then returned to
the nest at an age beyond which fratricide is likely.
As for the techniques used to demographic supplementation of wild raptors:
3 Breeding andNesting Biology inBirds ofPrey

84
Cross-fostering consists of placing young of one species into the nest of another
species. Many raptor species have been cross-fostered, either in captivity or in the
wild. There always is a risk though that cross-fostered individuals will become
imprinted upon the surrogate parental species. This may increase predation risk of
one of the species to the other (e.g. cross-fostering of peregrines on goshawk nests)
or hybrid production between the two species (e.g. greater spotted eagle and lesser
spotted eagle).
Hacking the controlled release of young raptors into the wild, is the most fre-
quently used technique to reintroduce or augment raptor populations. Nestling rap-
tors raised in captivity or in wild nests are translocated alone or in small groups of
three to ve individuals to the hacking site. The hacking site generally consists of a
wooden or metal tower with a large enclosure at the top constructed in such a way
as to provide the birds with a view of their surroundings (Fig.3.6). For some time,
individuals are fed in the enclosure, without seeing their handlers. At about the
natural edging time for the species, the front of the enclosure is opened, and the
birds inside have the opportunity to y freely and explore the surroundings. Food
continues to be provided in the enclosure for some time after it has been opened,
and released individuals often stay in the area for weeks or months before dispers-
ing or migrating. This technique has been successful with different breeding threat-
ened raptor populations (Negro etal. 2007; Gil etal. 2013; Sorenson etal. 2017).
Fig. 3.6 Hacking technique used in a recovery management plan of a threatened golden eagle
(Aquila chrysaëtos) population. (Photo credit: GREFA & Estación Biolóxica do Xurés (EBX))
L. Tapia and I. Zuberogoitia

85
Supplemental feeding may be used to increase raptor breeding success, and it also
can be used to increase productivity in poor-quality habitat (Rooney etal. 2015).
Scavenging species have been supplemented with food more often than has been
done with predatory raptors. Often used to enhance populations of carrion eaters
such as vultures, such stations have been referred to as “vulture restaurants” (Negro
etal. 2007). This management technique also has been used with territorial endan-
gered raptor species (González and Margalida 2008).
Nest-guarding the goal is to protect the nests of target species from depredation by
both wildlife and humans as well as from natural disturbances by actively monitor-
ing individual nests. An alternative management strategy is to establish buffer zones
around raptor nests aimed at protecting nests from the effects of recreational activi-
ties, forestry management activities, human development, etc. (Negro etal. 2007;
Zuberogoitia etal. 2014).
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