Inter-insular variation of the diet of Osprey Pandion haliaetus in the Canarian archipelago

Abstract

We studied the diet of the osprey Pandion haliaetus in the Canary Islands during 1997-2008 using prey remains under perches and nests, and direct observations. We collected data both in breeding territories and in non-breeding areas. We counted a minimum of 307 fish individuals as prey remains (both during breeding and non-breeding seasons), and identified another 78 during 433 hours of field observations. According to our results, ospreys consumed at least 15 taxa belonging to 12 families. We found slight differences in the spatial (both intra and inter insular) and temporal diet composition. During the breeding season, the main prey species were flying fishes (belonging to the family Exocoetidae) and needlefishes (belonging to the family Belonidae) according to the two employed methods (i.e. prey remains and direct observations). In the non-breeding period, the diet was composed primarily of non-autochthones freshwater fishes such as common carp Cyprinus carpio and goldfish Carassius auratus. In general, the diet diversity was similar to the diversity reported in other breeding populations of subtropical areas, and being less diverse than those of tropical areas. More precise studies evaluating the effect of fish availability in marine reserves, overfishing areas or fish farms on the demographic parameters are necessary for the management and conservation of threatened Canarian ospreys.

Full text

Original article
Wildl. Biol. 17: 240-247 (2011)
DOI: 10.2981/10-004
ÓWildlife Biology, NKV
www.wildlifebiology.com
Inter-insular variation of the diet of osprey Pandion haliaetus in the
Canarian archipelago
Manuel Siverio, Beneharo Rodrı
´guez, Airam Rodrı
´guez & Felipe Siverio
We studied the diet of the osprey Pandion haliaetus in the Canary Islands during 1997-2008 using prey remains under
perches and nests, and direct observations. We collected data both in breeding territories and in non-breeding areas. We
counted a minimum of 307 fish individuals as prey remains (both during breeding and non-breeding seasons), and
identified another 78 during 433 hours of field observations. According to our results, ospreys consumed at least 15 taxa
belonging to 12 families. We found slight differences in the spatial (both intra and inter insular) and temporal diet
composition. During the breeding season, the main prey species were flying fishes (belonging to the family Exocoetidae)
and needlefishes (belonging to the family Belonidae) according to the two employed methods (i.e. prey remains and
direct observations). In the non-breeding period, the diet was composed primarily of non-autochthones freshwater
fishes such as common carp Cyprinus carpio and goldfish Carassius auratus. In general, the diet diversity was similar to
the diversity reported in other breeding populations of subtropical areas, and being less diverse than those of tropical
areas. More precise studies evaluating the effect of fish availability in marine reserves, overfishing areas or fish farms on
the demographic parameters are necessary for the management and conservation of threatened Canarian ospreys.
Key words: Belonidae, Canary Islands, diet, direct observations, Exocoetidae, osprey, Pandion haliaetus, prey remains
Manuel Siverio, Constitucio
´n 17-3, E-38410 Los Realejos, Tenerife, Canary Islands, Spain - e-mail: mansiverio@
telefonica.net
Beneharo Rodrı´guez, La Malecita s/n, E-38480 Buenavista del Norte, Tenerife, Canary Islands, Spain - e-mail:
benerguez@terra.es
Airam Rodrı´guez, Department of Evolutionary Ecology, Estacio
´n Biolo
´gica de Don
˜ana (CSIC), Avda. Ame
´rico Vespucio
s/n, E-41092 Seville, Spain - e-mail: airamrguez@ebd.csic.es
Felipe Siverio, Los Barros 21, E-38410 Los Realejos, Tenerife, Canary Islands, Spain - e-mail: felipe.siverio@telefonica.net
Corresponding author: Beneharo Rodrı´guez
Received 12 January 2010, accepted 2 May 2011
Associate Editor: Anne Loison
The osprey Pandion haliaetus is a top predator spe-
cialised on fish. Its diet composition is greatly in-
fluenced by seasonal and geographical fish prey
distributions (Poole 1994). Several methods have
been employed to assess its diet worldwide: direct or
video recording observations of foraging birds or
fish deliveries to the nests (Ha
¨kkinen 1977, Edwards
1988, Eriksson 1988, Chubbs & Trimper 1998,
Glass & Watts 2009), identification of prey remains
collected under nests or feeding perches (Ha
¨kkinen
1978, Swenson 1978, Carss & Brockie 1994, Gil-
Sa
´nchez 1995, Fisher et al. 2001, Cartron & Molles
2002, Clancy 2005) or the combination of both
(McKlein & Byrd 1991, Carss & Godfrey 1996). As
only flesh and easily broken bones are ingested,
pellets are not useful for assessing osprey diet
(Francour & Thibault 1996).
Breeding sites of osprey in the Macaronesian
archipelagos (i.e. in the northeastern Atlantic) are
currently limited to the Canary and the Cape Verde
Islands, where 14 and ca 80 pairs occur, respectively
(Palma et al. 2004, Siverio 2008). Despite its delicate
conservation status (catalogued as Critically En-
dangered by the Red List of Spanish birds; Triay &
Siverio 2004), specific ecological aspects of the
Canarian population remain poorly known, with
240 ÓWILDLIFE BIOLOGY 17:3 (2011)

only a few surveys conducted regarding status,
distribution and breeding parameters (Siverio &
Rodrı
´guez 2007 and references therein). The avail-
able quantitative information on the food habits of
osprey in Macaronesia is limited to the Cape Verde
population (de Naurois 1987, Den Hartog 1990,
Ontiveros 2003, Martins 2006), with a few isolated
observations on the subject from the Canarian
population (Martı
´n & Lorenzo 2001). In our study
we quantify, for the first time, composition and
inter-insular variation of the osprey diet in the
Canary Islands, mainly during the nesting period.
We do this by prey remains analysis (PRA) com-
pared with direct observations (DO) on foraging
birds and birds delivering fish to the nests.
Material and methods
The Canarian archipelago (278-298N and 138-188W)
is located 96-100 km from the northwestern Atlantic
coast of Africa. It is composed of seven major
islands and some small islets and rocks. The current
osprey breeding distribution comprises the islands
of Lanzarote (including its related islets Montan
˜a
Clara and Alegranza belonging to the Chinijo
archipelago), Tenerife, La Gomera and El Hierro
(Fig. 1). Some individuals are regularly observed in
the remaining islands, but no successful breeding
attempts have been recorded there (Siverio & Ro-
drı
´guez 2007).
We assessed diet composition during breeding
(i.e. January-July; Siverio 2006) in 2003-2007, by vi-
Figure 1. Current breeding distribution of osprey in the Canarian archipelago (occupied islands are shown in black and the numbers of
breeding pairs are also given according to Siverio 2008). The histogrammes show the prey frequencies and sample sizes (N) on the three
studied islands.
ÓWILDLIFE BIOLOGY 17:3 (2011) 241

siting 22 perch or nest sites (five in Chinijo islets in
North Lanzarote, 12 on Tenerife and five on La
Gomera; see Fig. 1) of 12 of the 14 current breeding
territories of the archipelago (Siverio 2008), and by
collecting prey remains. To avoid disturbances, we
mainly collected prey remains during July-August,
just after fledglings leave the nests. We also collected
material at non-breeding sites (one on Gran Ca-
naria and two on Tenerife), where local breeding
adults were regularly sighted, but also European
visitors (colour-ringed) have been recorded (M.
Siverio & B. Rodrı
´guez, pers. obs.). This non-
breeding season material was presented and ana-
lysed independently. We placed emphasis on key
fish parts that provided taxonomic identification,
and we considered only fresh prey remains (i.e.
estimated to be ,2 months old). We assessed the
minimum number of fish individuals in prey
remains based on the most commonly found fins,
whole tails, jaws, different bones or body parts
representing an individual (Marti et al. 2007).
Whenever possible, we identified prey items at the
species level using a reference collection and fish
guides (Whitehead et al. 1986, Fischer et al. 1987,
Gonza
´lez et al. 2000, Brito et al. 2002, Miranda &
Escala 2002). We estimated sizes and weights of
common carp Cyprinus carpio according to formu-
las relating opercula size and the measurements
published by Gil-Sa
´nchez (1995) referred to a pop-
ulation of the Iberian Peninsula. For the other prey
species identified in our study, similar formulas were
not available in the literature. We also tried to
identify all fishes delivered to nests and being carried
by flying birds, using binoculars and telescopes (10-
60 magnifications), during the 1997-2008 breeding
seasons. For this phase of our study, we employed a
total of 433 observation hours at different breeding
territories and its nearest feeding areas (mainly
located on Tenerife and La Gomera). As it has been
reported that estimating length size from direct
observations entails important biases (Carss &
Godfrey 1996), we did not consider this direct
method to estimate the size of prey. Niche breath
and diet diversity were calculated using the stan-
dardised Levin’s (B
sta
) and Shannon (H’) indexes
(Krebs 1999) applied to the items consumed. The
Levin’s index formula is:
B¼1=Xp2
i;
where p
i
is the frequency of each food category
consumed. The standardised Levin’s index formu-
la is:
Bsta ¼B-1
Bmax -1;
where B is the Levin’s index and B
max
is the total
number of food categories recognised (lowest
niche breadth ¼0 and greatest niche breadth ¼1).
The Shannon index (in which higher values refer
to higher diversity) formula is:
H’¼-Xpilogpi:
We measured the diet overlap between islands using
percentage of food items through the Pianka’s index
(O):
Ojk ¼X
n
i¼1
pijpik
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X
n
i¼1
p2
ij X
n
i¼1
p2
ik
s;
where p
i
is the percentage of prey item ’i’in the diet
of islands ’j’and ’k’. Pianka’s index varies between 0
(total separation) and 1 (total overlap).
To study insular variation of the most important
prey (belonging to the families Exocoetidae and
Belonidae), we applied likelihood ratio tests (G-
tests), comparing a certain prey item with the total
number of the remaining prey items. We conducted
analyses using SPSS (version 17.0).
Results
During the breeding season, we counted a minimum
of 262 and 78 fish individuals in prey remains and
direct observation, respectively, including at least 15
taxa (belonging to 12 families; Table 1). The most
frequently consumed prey was flying fishes (belong-
ing to the family Exocoetidae; 42.7 and 19.2%
according to PRA and DO, respectively) and nee-
dlefish (belonging to the family Belonidae; 38.5 and
6.4% according to PRA and DO, respectively). At
least six species were represented by only one in-
dividual in the PRA and DO (see Table 1). We found
some remains of red rock crabs Grapsus grapsus at
two feeding perches on Tenerife, but they were not
considered in the analysis as we were not confident
that they were consumed by ospreys. Diet compo-
sition varied slightly between the islands (La Go-
mera: B
sta
¼0.35 and H’¼0.53, Tenerife: B
sta
¼0.21
and H’¼0.59 and Chinijo: B
sta
¼0.42 and H’¼0.63;
see Fig. 1). Only on Tenerife, fresh-water fishes were
caught, and the percentage of captures of flying
242 ÓWILDLIFE BIOLOGY 17:3 (2011)

fishes (belonging to the family Excoetidae) in Chi-
nijo was lower than on La Gomera and Tenerife (see
Fig. 1). Diet overlap (O), expressed as Pianka’s in-
dex, between La Gomera and Tenerife was 0.99, be-
tween Tenerife and Chinijo 0.66 and between Chi-
nijo and La Gomera 0.68. The two most important
groups of prey showed different patterns. Thus, the
species belonging to the family Exocoetidae were
significantly more consumed on Tenerife and La
Gomera than in Chinijo (G
2
¼20.85, P ,0.001), but
for species belonging to the family Belonidae it did
not vary between the islands (G
2
¼2.33, P ¼0.31).
In the non-breeding areas (all associated with large
artificial ponds), all identified prey was freshwater
fishes. Of a total of 45 fish individuals, the common
carp (belonging to the family Cyprinidae; 71.1%,
N¼32) was the most common species followed by
goldfish Carassius auratus (belonging to the family
Cyprinidae; 8.9%, N ¼4), tilapia Oreochromis
mossambicus (belonging to the family Cichlidae;
6.7%, N ¼3) and large mouth bass Micropterus
salmoides (belonging to the family Centrarchidae;
2.2%, N ¼1), whereas the rest remainded uniden-
tified (11.1%, N ¼5). Estimated mean size and
weight of common carp (N¼35) captured by osprey
were 25.9 cm 62.6 (range: 21.5-32.3) and 223.3 g 6
43.4 (range: 147.4-329.3), respectively (Fig. 2).
Discussion
We found only small differences between prey re-
main analysis and direct observations (see Table 1),
so both methods could be considered complemen-
tary, though some limitations and biases are present
(see Marti et al. 2007). Direct observation is time
consuming, and the percentage of correct identifi-
cations is highly biased and related to several factors
such as size, colour and morphology of the fish, and
by the distance of the observation (Carss & Godfrey
1996). Fish identification is also impeded, because
often fish are partially eaten when observations
commence. In the case of prey remains analysis,
some biases are associated with identifiable, con-
spicuous and lasting body pieces, so usually the
smallest fish are more prone to be undetected
compared to bigger fish (Carss & Brockie 1994).
The presence of scavengers that may affect the
durability of larger prey remains, could represent a
Table 1. Breeding diet composition of osprey in the Canary Islands according to prey remains analysis (PRA) and direct observations
(DO) during the periods 2003-20 07 and 1997-2008, respectively (see details in text). MNI ¼Minimum number of individuals.
Prey taxa PRA DO
Family Species MNI % MNI %
Marine fish
Cupleidae Unidentified 1 4 - -
Mugilidae Unidentified 1 4 1 1.3
Belonidae Tylosurus acus* 96 366 5 6.4
Belonidae Unidenfied 5 19 - -
Exocoetidae Cheilopogon heterurus* 112 427 15 19.2
Aulostomidae Aulostomus strigosus - - 1 1.3
Moronidae Dicentrarchus labrax - - 1 1.3
Carangidae Trachinotus ovatus 5 19 2 2.6
Sparidae Sparus auratus 4 15 2 2.6
Sparidae Diplodus sp. 2 8 - -
Scaridae Sparisoma cretense 415- -
Sphyraenidae Sphyraena viridensis 14--
Freshwater fish
Cichlidae Oreochromis mossambicus 14--
Cyprinidae Carassius auratus - - 7 9.0
Cyprinidae Cyprinus carpio 10 38 - -
Unidentified Unidentified 20 76 44 56.4
Total 262 78
B
sta
0.15 0.38
H’ 0.55 0.70
* includes primarily this species but possibly others too.
ÓWILDLIFE BIOLOGY 17:3 (2011) 243

small bias in our study as we collected remains at the
end of the nesting season. In this sense, we noticed
that some prey remains were probably moved into
cracks by the black rat Rattus rattus. Furthermore,
yellow legged gull Larus michahellis and common
raven Corvus corax usually occur in the vicinity of
the osprey feeding perches or nests, apparently
searching for food (M. Siverio & B. Rodrı
´guez, pers.
obs.).
Taking into account the quantitative data from
our study and the addition of saddled seabream
Oblada melanura (belonging to the family Spari-
dae), quoted as an occasional item (Martı
´n&
Lorenzo 2001), the diet of osprey in the Canary
Islands is composed of a minimum of 16 fish taxa
(belonging to 12 families). This diet diversity is
comparable to reports from other Palearctic popu-
lations, but lower than reported in tropical waters
such as the southern Red Sea or Cape Verde (Table
2). Reviewing osprey dietary studies during breed-
ing in the western Palearctic, northern populations
feed mainly on freshwater fishes, while southern
populations feed mainly on marine fishes, and it
seems that diet diversity increases in a latitudinal
gradient southward (see Table 2). It has been
suggested that sea surface temperature is the main
proximate factor affecting surface fish availability
(both factors related positively), and consequently,
osprey feeding behaviour selection into marine or
freshwater environments (Marquiss et al. 2007).
However, the proportion of marine items in the
osprey diet must also be influenced by local factors
such as availability of foraging areas (e.g. freshwa-
ter body masses are scarce and small on the Canary
Islands) and human disturbances. In this sense, it is
well-known that ospreys are generalist and oppor-
tunistic foragers on fish, depending greatly on
locally available resources (Poole 1994, Martins
2006). This behaviour also explains the observed
spatial differences within the Canarian archipelago.
According to the Pianka’s index, the diets of ospreys
from La Gomera and Tenerife overlap more (0.99)
than the osprey diet from Chinijo compared with La
Gomera (0.68) or Tenerife (0.66). These differences
are probably related to fish availability as Canarian
coastal fish assemblages vary greatly within and
between the islands according to the particular
habitat features and human pressure (Falco
´n et al.
1996, Tuya et al. 2004, Clemente et al. 2010).
During the breeding season, freshwater fishes
(belonging to the families Cichlidae and Cyprinidae)
were only consumed in low numbers on Tenerife (see
Figure 2. Estimated size (A) and weight (B)
of common carps (N ¼35) consumed by
osprey on the Canary Islands based on the
opercula size according to equations de-
scribed in Gil-Sa
´nchez (1995).
244 ÓWILDLIFE BIOLOGY 17:3 (2011)

Fig. 1), and thus the relative high level of diet diversity
there (H’¼0.59) could be related to their consump-
tion. Both on Tenerife and La Gomera, although
more abundant on Tenerife (Siverio et al. 2008, M.
Siverio & B. Rodrı´guez, pers. obs.), the breeding
territories are situated close to water ponds used for
agriculture. Many of these reservoirs contain domes-
ticated fishes which make up potential prey for
ospreys. However, according to our observations on
Tenerife, the domesticated fishes seem to be con-
sumed mainly during the non-breeding season or by
non-breeding or migratory birds. Given the lack of
native freshwater fish in the Canary Islands, the
artificial fish source may be crucial when weather
conditions (strong winds or rough sea) impede fishing
at sea (Grubbs 1977). It is known that birds also
forage on fish farms of gilthead seabream Sparus
auratus and European seabass Dicentrarchus labrax
(M. Siverio & B. Rodrı
´guez, pers. obs.), of which
some are close to nesting sites of Tenerife.
Although several non-fish prey have been re-
ported worldwide (see Wiley & Lohrer 1973), in the
Canaries only lizards (two Caesar’s lizard Gallotia
caesaris individuals captured on El Hierro; Dı
´az et
al. 1986) and crabs (without any more information;
Martı
´n & Lorenzo 2001) have been recorded.
Curiously, recent observations have indicated that
some ospreys ingested algae (Rhizoclonium sp.
belonging to the family Cladophoraceae) and slime
near the banks of an agricultural water reservoir,
but the reason for this behaviour remains un-
known (Siverio et al. 2008).
The estimated mean length (25.9 cm) and weight
(223.3 g) of the common carp consumed by ospreys
in the Canaries are within the range recorded
elsewhere (Ha
¨kkinen 1978, Poole 1989, Francour
& Thibault 1996). It has been observed that the size
and weight of fish captured by ospreys vary ac-
cording to its availability, and it has been sug-
gested that neither species nor size are selected for
by this raptor (Swenson 1978, Poole 1989, Carss &
Godfrey 1996, Francour & Thibault 1996).
Our study indicates that during the breeding
season, the osprey diet in the Canaries is mainly
composed of flying fishes (belonging to the family
Exocoetidae) and needlefish (belonging to the
family Belonidae), species whose abundance is
probably related to their very low commercial
fisheries value (G. Gonza
´lez-Lorenzo, pers. com.).
The slightly higher diet diversity observed in the
Chinijo islets compared to Tenerife and La
Gomera could be influenced by the fact that the
former present a better conservation state of
coastal fish assemblages as they were declared as
a Marine Reserve by the Spanish Government in
1986. At this site, protection measures against
overfishing have contributed to the increase and/or
the maintenance of the populations of certain
heavily exploited species, such as for example the
parrot fish Sparisoma cretense (Garcı
´a-Charton et
al. 2008). The differences in the management re-
Table 2. Comparison of diet of selected osprey breeding populations across the western Palearctic based on prey remains. The Behaviour/
main foraging ground abbreviations are M-Fe¼migratory and freshwater environment and S-Me ¼sedentary and marine environment.
Location Latitude Longitude
Behaviour/main
foraging ground N8prey Species/taxa B
sta
H’Source
Finland (inland) 60.58N 23.88E M-Fe 716 12 0.13
a
0.58
a
Ha
¨kkinen (1978)
Finland (coastal) 60.38N 21.38E M-Fe 198 10 0.33
a
0.71
a
Ha
¨kkinen (1978)
Scotland (north) 56.68N 3.68W M-Fe 104 6 0.63 0.67 Carss & Brockie (1994)
Scotland (south) 54.98N 4.48W M-Fe 239 9 0.36 0.69 Marquiss et al. (2007)
Germany (north-east) 51.58N 13.58E M-Fe 562 6 0.47 0.22 Mu
¨ller et al. (2005)
France (south)
b
47.18N 2.58E M-Fe 90 14 0.60 1.02 Thiollay & Wahl (1998)
Corsica (inland)
c
41.88N 8.78E S-Me 258 12 0.46 0.87 Francour & Thibault (1996)
Portugal (south)
d
37.18N 8.68W S-Me 49 9 0.21 0.62 Cancela & Palma (1984)
Canary Islands 28.28N 15.38W S-Me 262 13
e
0.15 0.55 Our study
Red Sea (south) 16.88N 42.08E S-Me 688 56 0.30
f
- Fisher et al. (2001)
Cape Verde Islands 16.18N 22.88W S-Me 1264 32 0.20 1.01 Martins (2006)
a
calculated using number of key bones per species;
b
study based on direct observation;
c
considering also some direct observations;
d
currently extinct population;
e
16 considering prey remains analysis, direct observations and bibliographic sources (Martı
´n & Lorenzo 2001);
f
mean value of four studied islands.
ÓWILDLIFE BIOLOGY 17:3 (2011) 245

gimes could be affecting the diet composition of
osprey in Chinijo (a marine reserve) vs Tenerife
and La Gomera (unprotected and overfishing ar-
eas). More precise studies to assess the effect of
availability of fish (under natural conditions such
as marine protected areas, or at artificial sources
such as water ponds or fish farms) on the breeding
success and demographic parameters of popula-
tions are necessary for the management and con-
servation of threatened Canarian ospreys.
The use of fish farms as artificial sources of food
by osprey could also represent an additional source
of mortality as ospreys may entangle in the nets that
cover the cages (Siverio & Rodrı
´guez 2007). As it is
suspected that the maintenance staff of these farms,
to avoid legal problems, may easily hide dead birds
if found, competent authorities should inspect these
installations to detect and correct potential risks to
the raptor species.
Acknowledgements - we are very grateful to Francisco M.
Gonza
´lez for his interest in our study and for allowing us
access to some otherwise inaccessible locations using his
boat. We also thank Aurelio Acevedo, Leandro De Leo
´n
and Jaime Ginove
´s who helped us collect prey remains.
Special thanks go to Alejandro Sancho, Jose
´Carlos
Herna
´ndez and Alberto Brito of the Department of
Zoology (La Laguna University), and Rafael Miranda of
the Department of Zoology (Navarra University) for their
help in the identification of prey remains. Furthermore,
Juan Curbelo, Jose
´G. Martı
´n, Jose
´J. Herna
´ndez, Manuel
Pe
´rez and Rosme
´n Ramos provided fish for our reference
collection. The 2004-2005 surveys in Tenerife were
partially funded by Oficina de Gestio
´n del Parque Rural
de Teno (Cabildo de Tenerife). In addition, the Canarian
delegation of SEO/BirdLife supported us by covering
some of the field expenses. Mikael Hake and Daniel
Schmidt provided valuable information of osprey diet in
Sweden and Germany, respectively. Finally, Greg. P.
Clancy (University of New England, Armidale, NSW
Australia), Charles J. Henny (USGS, Forest and Range-
land Ecosystem Science Center, Oregon) and an anony-
mous referee provided valuable comments and improved
the English version of earlier drafts of the manuscript.
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