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Non-breeding feeding ecology of territorial Bonelli’s Eagles Hieraaetus fasciatus in the Iberian Peninsula

Authors:
The feeding ecology of many species of
raptors remains largely unknown since most
of diet studies are usuallyrestricted to a half
of the year, the breeding season, probably due
to the easiness for recovering food data relat-
ed to the association of individuals to nesting
sites. Contrary, during the non-breeding
season birds are difficult to locate and infor-
mation on diet is scarce, causing lack in over-
all knowledge and comprehension of feeding
habits (Cramp and Simmons, 1980; del
Hoyo et al., 1994; Ferguson-Lees and Christie,
2001). Because of food is one of the main lim-
iting factors for birds of prey (Newton, 1979),
this shortage in basic information during a
long life-period of such species should be ur-
gently addressed. In this sense, the diet of non-
breeding period have proved to influence the
healthy of birds, body condition and the repro-
ductive output in the subsequent breeding at-
tempt (Newton, 1979; González, 1991), final-
ly leading to a strong limitation of both den-
sity and survival of a number of bird species
(see a review in Newton, 1998). Hence the
study on non-breeding diet in raptors, a group
of species usually threatened (del Hoyo et al.,
1994; Tucker and Heath, 1994), is not only an
important aspect to promote the ecology
knowledge but also a necessary tool to plan
adequately conservation measures.
The Bonelli´s eagle Hieraaetus fasciatus is
an endangered bird of prey(Tucker and Heath,
1994; Real, 2004) inhabiting the Mediterranean
coast, Middle East and southern Asia (del Hoyo
et al., 1994; Ferguson-Lees and Christie, 2001).
Dietary studies on this species are frequently
related to the breeding season and restricted
around European continent (Jordano, 1981;
Palma et al., 1984; Fernández and Insausti,
1986; Real, 1987; Salvo, 1988; Simeon and
Wilhelm, 1988; Rico et al.,1990; Real, 1991;
Gil-Sánchez et al., 1994; Leiva et al., 1994;
NON-BREEDING FEEDING ECOLOGY OF TERRITORIAL
BONELLI´S EAGLES HIERAAETUS FASCIATUS
IN THE IBERIAN PENINSULA
ECOLOGÍA TRÓFICA DE LAS ÁGUILAS-AZOR PERDICERAS
HIERAAETUS FASCIATUS TERRITORIALES DURANTE EL PERIODO
NO REPRODUCTOR EN LA PENÍNSULA IBÉRICA
Marcos MOLEÓN*1,José María GIL-SÁNCHEZ**, Joan REAL***, José Antonio
SÁNCHEZ-ZAPATA****, Jesús BAUTISTA** and José Francisco SÁNCHEZ-CLEMOT*****
*Departamento de Biología Animal, Universidad de Granada, Granada, E-18071 Spain.
** Empresa de Gestión Medioambiental, Consejería de Medio Ambiente, Junta de Andalucía, Spain.
*** Departament de Biologia Animal, Universitat de Barcelona, Av. Diagonal 645, Barcelona,
E-08028 Spain.
**** Departamento de Biología Aplicada, Universidad Miguel Hernández, Ctra. de Beniel km 3.2,
Orihuela, Alicante, E-33012 Spain.
***** Consejería de Medio Ambiente, Junta de Andalucía, Spain.
1Corresponding author: mmoleonpaiz@hotmail.com
Ardeola 54(1), 2007, 135-143
Martínez et al., 1994; Gil-Sánchez, 1998; Gil-
Sánchez et al., 2000; 2004; Iezekiel et al., 2004;
Palma et al., 2006). Only few works exist
facing the non-breeding period (France: Chey-
lan, 1977; Simeon and Wilhelm, 1988; Cyprus:
Iezequiel et al., 2004), but these studies were
made using heterogeneous collecting methods
(Cheylan, 1977; Simeon and Wilhelm, 1988),
or were focused on non continental Bonelli´s
populations (Iezequiel et al., 2004).
The main goals of this paper were: 1) to de-
scribe the diet of territorial Bonelli´s eagles
during the non-breeding season in two repre-
sentative areas of the Iberian Peninsula, 2) to
study seasonal differences related to breed-
ing behaviour and 3) to discuss the profitabil-
ity of the main preyand prey preferences for
Bonelli´s eagles under the light of classic pred-
ator-prey theories.
Our research were focused in two different
areas representing very distinct ecological and
demographical patterns in the European
species distribution (Real and Mañosa, 1997;
Real et al., 2001; Muñoz et al., 2005; Car-
rascal, in press), one in the south (Granada
province; 37º20´N, 03º45´E) and other in the
northeast (Catalonia region; 41º34´N, 01º25´E)
of Spain. Granada, with ca. 52 Bonelli´s breed-
ing pairs and showing a slight increased ten-
dency, supports the most productive popula-
tion of this species in Europe (Gil-Sánchez
et al., 2004; Moleón and Gil-Sánchez,
2006), while Catalonia sustains a decreasing
population of ca. 66 pairs, even though during
the last years reached some stability (Real and
Mañosa, 1997; Real, 2004). The habitat of the
Bonelli´s eagle in Granada is mainlycharac-
terised by a mixture of non-irrigated crops and
Mediterranean shrubs, while the Catalonian
habitat is more forested.
In relation to the known trophic ecology (in
the breeding season), both areas are also clear-
ly differentiated, since Bonelli´s eagles in
Granada consume mainlyrabbits Oryctolagus
cuniculus and red-legged partridges Alectoris
rufa (both preymeaning ca. 70 % of the total
diet; Gil-Sánchez et al., 2000, 2004) while in
Catalonia pigeons and rabbits are the more fre-
quent prey (both meaning ca. 50 % of the to-
tal diet), existing a broader trophic spectrum
(Real, 1987, 1991).
We collected diet data searching for food re-
gurgitated pellets and freshly captured prey
in roosting sites in the breeding areas from the
end of dependence period of juveniles until egg
laying (October-January; Arroyo et al., 1995;
Real et al., 1998; Gil-Sánchez, 2000; Mínguez
et al., 2001). These two methods offer the most
reliable results for the diet composition in this
species (Real, 1996). The study period was
comprised between 1981-2002 for Catalonia
and 1998-2007 for Granada, corresponding to
14 and 9 breeding territories respectively.
Firstly, we contrasted the overall non-breed-
ing food habits from Granada and Catalonia,
and then we compared by territory the diet be-
tween breeding and non-breeding periods only
using territories with sufficient sample size (>
20 prey items) coinciding for the same years
(Granada: n=4; Catalonia: n=3). Breeding
samples were collected from February to June
(see Gil-Sánchez et al., 2000, 2004; Real, 1996;
for more methodological details).
Prey species were grouped in seven cate-
gories: rabbit, other mammals, red-legged par-
tridge, pigeons Columba spp., corvids, other
birds and eyed lizard Lacerta lepida.All diet
analyses were tested by means of a chi square
and assumed P< 0.05.
At a global level we obtained 519 prey items
for the non-breeding season, 412 from Grana-
da and 107 from Catalonia. By regions, in south-
ernSpain the rabbit was the staple prey(52 %),
followed bypartridge (18 %) and pigeons (16
%); the other preycategories were of minor im-
portance. In Catalonia the main prey was pi-
geons (49 %), followed by other birds (25 %);
rabbit reached only 10 % (Table 1).
The non-breeding diet composition differed
between Granada and Catalonia (
χ
2=40.2; df
=6; P<0.001), as rabbits and partridges were
more frequentlyconsumed in the south (
χ
2=
MOLEÓN, M., GIL-SÁNCHEZ, J. M., REAL, J., SÁNCHEZ-ZAPATA, J. A., BAUTISTA, J. and SÁNCHEZ-CLEMOT, J. F.
Ardeola 54(1), 2007, 135-143
136
10.2-60.5; df = 1; P<0.001 for both cases)
while pigeons and other birds being more nu-
merous in the north (
χ
2=21.6-51.0; df = 1; P
<0.001 for both cases; Table 1).
We also found differences between breed-
ing and non-breeding diet in five of the seven
territories (“Granada 3-4” and “Catalonia 1-
3”;
χ
2=18.8-68.6; df = 6; P<0.01 for all cas-
es; Table 2; Fig. 1). Overall, in Granada the
consumption of pigeons were higher in the non-
breeding season (
χ
2=7.3; df = 1; P<0.01),
while partridges fell off at half (
χ
2=20.6; df
=1; P<0.001); the other prey groups did not
suffer seasonal changes (P>0.05 for all cas-
es; Table 2; Fig. 1). In Catalonia there was a
higher ingest of pigeons and other birds (
χ
2=
7.0-17.2; df = 1; P<0.01 for both cases), and
alower consumption of rabbit, other mammals
and lizards in the non-breeding period (
χ
2=
5.7-10.2; df = 1; P<0.05 for all cases); par-
tridges and corvids were equallyconsumed
in both periods (P>0.05 for both cases;
Table 2; Fig. 1).
Independently of the season, rabbit was the
main prey of the Bonelli´s eagle in southern
Spain. Taking into account that rabbit abun-
dance decreases over ca. 50 – 80 % in the ea-
gle non-breeding season in relation to the breed-
ing period (Villafuerte et al., 1997; Gil-Sánchez
et al., 1999; Calzada, 2000; Mínguez et al.,
2001; Palomares, 2001), the absence of sea-
sonal changes in the rabbit consumption rate
suggests a functional response and an active
selection by Bonelli´s eagle to this prey in
Granada (Stephens and Krebs, 1986), as oth-
er studies had previously described (Gil-
Sánchez, 1998; Palma et al., 2006). Then, the
higher mobility of eagles during the non-breed-
ing season (Consejería de Medio Ambiente,
2006; J. Real and M. Moleón, unpubl. data of
radiotracking) due to the lack of parental care
could favour the displacements and staying in
rabbit high density patches, although being
these areas far away from the nests, so giving
aplausible explanation to our results.
The scenery in Catalonia is different, since
in this region the rabbit is on average much
less abundant than in southernSpain (Real,
1991; Blanco and Villafuerte, 1993; Villa-
fuerte et al., 1998; Gil-Sánchez et al.,
2004), consequentlybeing less frequent in the
eagle diet than in Granada. In fact the winter
rabbit scarcity is so low that functional re-
sponse could remain hidden. So it is likely
that rabbits lost their profitability for eagles
in these verylowrabbit density areas and sea-
sons, and they were permuted byother prey
likepigeons and other birds.
Ardeola 54(1), 2007, 135-143
NON-BREEDING FEEDING ECOLOGY OF BONELLI´S EAGLES 137
TABLE 1
Average food habits of the Bonelli´s eagle in southern (Granada province) and northeastern (Catalonia re-
gion) Spain during the non-breeding season. Data are referred to frequencies of occurrence, and values
providing differences (P<0.05) are given in bold.
[Alimentación del águila-azor perdicera en el sur (provincia de Granada) y el noreste (región de Cata-
luña) de España en época no reproductora. Los datos se refieren a frecuencias de ocurrencia. Los valo-
res que aportan las diferencias (P<0,05) se señalan en negrita.]
Other Other
Geographic area Rabbit mammals Partridge Pigeons Corvids birds Lizard n
Southern Spain 51.7 3.9 17.7 16.0 1.5 7.8 1.5 412
Northeastern Spain 9.8 3.9 4.9 49.0 7.8 24.5 0 107
The Bonelli´s eagle is the Iberian predator
consuming more red-legged partridge (see a
review in Moleón, 2007), which is an ex-
pected result taking into consideration that this
species belongs to the hawk eagles, a group of
raptors including a high number of galliforms
in their diet (Brown, 1952, 1955; Smeenk,
1974; Steyn, 1975; Debus, 1984; Nevado et
al., 1988; Martínez, 2002; García-Dios, 2006);
besides, some authors havesuggested cer-
tain ornithophagical specialization (Clouet
and Goar,1984; Parellada et al., 1984). Sur-
prisingly we did not find evidences of any func-
tional response to this prey, so that the propor-
tion of partridges in the Granada diet
diminished when increased their abundance
in the field (Braza et al.,1985; Duarte and Var-
gas, 2001; Mínguez et al., 2001). The lower
consumption of partridge during the non-
breeding period could be related to seasonal
differences in its vulnerability to predators. In
this respect, the exhibition calls by males of
partridges in spring (Cramp and Simmons,
1980) could make themselves easier to catch
by eagles in the breeding season (pers. obs.),
as it has already been suggested in the case of
another Iberian raptor (Donázar and Castién,
1989). For its part, the seasonal stability of
red-legged partridges in the diet of Bonelli’s
eagle in Catalonia could be a functional re-
sponse to the non-breeding scarcity of a more
profitable prey such as rabbit. The lower avail-
ability of rabbits in this area should therefore
force eagles to preyon partridges in the non-
breeding period,so compensating for their low-
er vulnerability.
MOLEÓN, M., GIL-SÁNCHEZ, J. M., REAL, J., SÁNCHEZ-ZAPATA, J. A., BAUTISTA, J. and SÁNCHEZ-CLEMOT, J. F.
Ardeola 54(1), 2007, 135-143
138
TABLE 2
Comparative diet of the Bonelli´s eagle during the breeding and non-breeding seasons in four territories
of Granada (southern Spain) and three territories of Catalonia (northeastern Spain). Frequency of occur-
rence of each prey group is shown.
[Dieta comparada del águila-azor perdicera en los periodos reproductor y no reproductor en cuatro te-
rritorios de Granada (sur de España) y tres de Cataluña (noreste de España). Se muestra la frecuencia
de ocurrencia de cada grupo-presa.]
Other Other
Territory Period Rabbit mammals Partridge Pigeons Corvids birds Lizard n
Granada 1 Non breed. 36.2 10.3 15.5 19.0 5.2 10.3 3.4 58
Breeding 39.6 2.1 24.7 16.6 4.2 5.6 7.1 889
Granada 2 Non breed. 64.7 2.5 19.6 4.5 1.6 6.7 0.5 231
Breeding 47.7 7.5 29.0 5.5 1.4 6.7 2.3 656
Granada 3 Non breed. 40.9 6.8 20.5 22.7 0 6.8 2.3 44
Breeding 22.3 1.5 39.3 18.0 1.0 5.3 12.6 206
Granada 4 Non breed. 30.3 0 6.1 48.5 0 9.1 6.1 33
Breeding 43.3 3.3 13.3 30.0 0 10.0 0 30
Catalonia 1 Non breed. 15.0 0 10.0 10.0 15.0 40.0 0 20
Breeding 23.5 15.9 627.2 610.1 11.3 455
Catalonia 2 Non breed. 10.7 3.6 3.6 50.0 7.1 25.0 0 28
Breeding 17.6 19.0 1.4 33.1 3.5 7.0 18.3 142
Catalonia 3 Non breed. 4.5 0 0 68.2 9.1 18.2 0 22
Breeding 25.3 21.3 6.7 29.3 6.7 6.7 4.0 75
The overall relative scarcity of rabbits and par-
tridges in Catalonia (Real, 1991; Villafuerte et
al., 1993; Villafuerte et al., 1998; Gil-Sánchez
et al., 2004) could be the cause of the differences
observed between areas, so that northern eagles
were forced to capture alternative prey (Angel-
stam et al., 1985), namely pigeons, other birds,
other mammals and lizards. In relation to the
non-breeding season, the lack in rabbits in north-
ern Spain is probably accentuated by the lower
availability of cold-sensitive species like lizards
(Pérez-Mellado, 1998) and certain other mam-
mals (e.g., Sciurus vulgaris;Blanco, 1998), lead-
ing to a higher consumption of pigeons (main-
lydomestic) and species included into the
other birds´ category, which are more abundant
in this season due to the arrival of wintering birds
(Díaz et al., 1996; Tellería et al., 1999).
To conclude, non-breeding diet suggested
that rabbits are a keypreyfor Bonelli´s ea-
gles in the Iberian Peninsula, although rather
than a true trophic specialist, the Bonelli´s ea-
gle can be considered as a facultative special-
ist (Glasser, 1982), preferring the rabbit
when it is relatively abundant but shifting to
other prey when the rabbit is too scarce, which
is neither a surprise nor an exception for the
Mediterranean community of vertebrate pred-
ators (Fedriani et al., 1998; Calzada, 2000;
Lozano et al., 2006). This situation would fit
with the sigmoidal Type III functional response
(Holling, 1959), and is therefore not consistent
with the specialist Type II found in southern
Portugal by Palma et al.(2006). Twohy-
potheses can provide a suitable explanation at
this respect. Firstly,it might be that rabbit pop-
ulation density in southern Portugal is not un-
der the profitability threshold, as it seems to be
the case of Granada. Secondly, it is possible
that differential food requirements and avail-
ability among seasons lead to differential strate-
gies of resource exploitation byBonelli´s ea-
gles. Consequently,further research is needed
taking into account different ecological scener-
Ardeola 54(1), 2007, 135-143
NON-BREEDING FEEDING ECOLOGY OF BONELLI´S EAGLES 139
FIG.1.—Average food habits of the Bonelli´s eagle in four territories of southern Spain (a) and three ter-
ritories of northeastern Spain (b) having data from both breeding and non-breeding periods. Black bars:
non-breeding period; white bars: breeding period; RAB: rabbit; O M: other mammals; PAR: red-legged
partridge; PIG: pigeons; COR: corvids; O B: other birds; LIZ: eyed lizard. Asterisks indicate prey
groups providing the statistical differences (P< 0.05).
[Dieta del águila-azor perdicera en cuatro territorios del sur (a) y tres territorios del noreste de España
(b) que cuentan con datos para las épocas reproductora y no reproductora. Barras negras: periodo no re-
productor; barras blancas: periodo reproductor. RAB: conejo; O M: otros mamíferos; PAR: perdiz roja;
PIG: palomas; COR: córvidos; O B: otras aves; LIZ: lagarto ocelado. Los asteriscos señalan los grupos-
presa que aportan las diferencias (P< 0,05).]
ies and seasons where both the eagle diet and
the rabbit densities are known.
Nonetheless, the searching for the richer non-
breeding prey patches can be a concern for the
eagle conservation. In this sense eagles with low-
er trophic resources as rabbits have bigger home
ranges and have to move long distances outside
the breeding season (pers. obs.)that implies a
supplementary energy expense. Moreover, high
rabbit density areas support a strong game ac-
tivity during the non-breeding season, and ille-
gal persecution by hunters is one of the two main
mortality causes for Bonelli´s eagles (Real et al.,
2001). On the other hand,pigeons and other birds
like gulls, thushes and corvids are usually asso-
ciated to humans, which increase the mortality
risk bydirect persecution, power lines casual-
ties, parasite diseases and poisoning. Hence,
Bonelli´s eagle conservation planning should
take into account this prey-mediated vulnerabil-
ity, even more because this raptor frequently
breeds and disperses in human-influenced habi-
tats (Gil-Sánchez et al., 1996; Bautista et al.,
2004; Gil-Sánchez et al., 2004; Balbontín, 2005).
RESUMEN.Presentamos los primeros da-
tos sobre la dieta de las águilas-azor perdi-
ceras Hieraaetus fasciatus territoriales duran-
te el periodo no reproductor en la península
Ibérica. El estudio, realizado en dos áreas, una
del sur (Granada) y otra del noreste (Catalu-
ña) de España, mostró que existen diferencias
alimenticias tanto geográficas como estacio-
nales. El águila parece comportarse como un
especialista facultativo sobreel conejo,de ma-
neraque prefiereesta presa cuando es relati-
vamente abundante perodesvía su atención
hacia presas alternativas cuando el conejo es
demasiado escaso.El consumo de perdices ro-
jas parece estar condicionado por la abundan-
cia de conejo y el conspicuo comportamiento
de los machos de perdiz durante el celo. En los
lugares y épocas donde los conejos y, en me-
nor medida, las perdices están menos dispo-
nibles paralas águilas, presas como las pa-
lomas y otras aves adquieren especial relevan-
cia. Las preferencias alimenticias y las restric-
ciones impuestas por la disponibilidad de las
presas incrementan la vulnerabilidad de las
águilas-azor perdiceras hacia amenazas de
origen antrópico, circunstancia que se ve acen-
tuada en época no reproductora.
ACKNOWLEDGEMENTS.—We acknowledge the
kind collaboration of Isidro Moleón, Lourdes
Moleón and Sebastián Justicia in the field work.
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[Recibido: 09-04-07]
[Aceptado: 20-06-07]
Ardeola 54(1), 2007, 135-143
NON-BREEDING FEEDING ECOLOGY OF BONELLI´S EAGLES 143
... The study area is intermingled with nonirrigated and irrigated cultivation zones. Prey species for the Bonelli's eagle include partridges (Alectoris rufa), pigeons (Columba spp.), rabbits (Oryctolagus cuniculus), hares (Lepus granatensis), and lizards (Timon spp.), among others (e.g., L opez-L opez & Urios, 2010; Mole on et al., 2007, Mole on, S anchez-Zapata, et al., 2009Ontiveros & Pleguezuelos, 2000;Ontiveros et al., 2005), and all of them are present in the different landscapes of the study area. ...
... Shade shows the 95% CI of the predicted effect. (Mole on et al., 2007;Ontiveros et al., 2005). Thereby, GPS information validated the results obtained by the approach we made to make data comparable among territories. ...
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Species life history and anthropogenic influence are important drivers of population performance and viability in human‐dominated ecosystems. How these factors affect habitat selection and occupancy in long‐lived species is an important topic for their conservation. Long‐term datasets are needed for establishing the underlying drivers of this process. In this 22 year‐long study, we conducted annual surveys of Bonelli's eagle in the east of the Iberian Peninsula. During this period, 42.8% of the known territories remained unoccupied. Territories with a higher likelihood of raising two chicks over time were stable, evidenced by a lower coefficient of variation in productivity, and were more likely to remain occupied. Moreover, territories with lower habitat diversity, dominated by coniferous forest or agricultural fields, and those located further away from the coast and at higher altitudes showed lower rates of occupancy (i.e., unoccupied >3 consecutive years). To validate these associations, we monitored space use of 22 individuals equipped with Global Positioning System/Global System for Mobile (GPS/GSM) transmitters, which confirmed that eagles selected for open habitats (mainly scrublands and transitional woodland‐scrubs) intermixed with forest areas within their home ranges. In contrast, individuals avoided areas dominated by agricultural, urban, and continuous forests for breeding in line with the observations for unoccupied territories. Our results highlight the important interplay between natural and anthropogenic factors, which also have important implications for other raptor species. Preservation of the most productive territories and the re‐occupancy of unoccupied territories along with reducing threats in the preferred habitats are fundamental actions that should be taken immediately to sustain viable populations. Potential management actions include enhancing natural prey density through habitat restoration and conservation, mitigating threats and reducing mortality risks due to power lines, fences, poisoning, and maintaining habitat heterogeneity important to eagles' hunting activities.
... [24][25][26][27] In Andalusia, Bonelli's Eagles prey mainly on rabbits Oryctolagus cuniculus and Red-legged Partridges Alectoris rufa, while in Catalonia their diet is more diverse and ornitophagous. [28][29][30] Bonelli's Eagles in Andalusia exhibit higher pair density, productivity and adult survival than in Catalonia. 19 Tables 1 and 2) frequently used in morphometry studies on birds of prey. ...
... Interpopulation variation in discriminant characteristics was also found for immature, subadult and adult Bonelli's Eagles, 16 as well as in other raptor species [51][52][53] and other birds. 54 The differences between subpopulations in body mass and claws and tarsus length could be related to the different diet of Catalonian and Andalusian Bonelli's Eagles, 28,29,55,56 or to some other environmental variable that differs between the two subpopulations. Whatever the cause, these differences between Catalonian and Andalusian birds are likely a bit masked by the continuous gene flow between the studied subpopulations, 19 but still suggest a certain genetic structure within the Iberian population of Bonelli's Eagle, which has been found to be relatively homogeneous in genetic terms. ...
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... Whenever predators feed upon two prey species, selectivity or preference based predation of these species plays a significant role in the system dynamics [25]. An extended situation of Europe, the different prey species availability that could be in turn 'optimal' lead to expect a Holling type III response as suggested by Moleon et al. [26]. In this paper, we have presented and compared the simulation results of the spatial model with Holling type II and III functional responses. ...
... Moleon et al. [26] have suggested that the Bonelli´s eagle can be considered as a facultative specialist [27] and preferring the rabbit when it is relatively abundant. But shifting to other prey when the rabbit is too scarce, that situation would fit with the sigmoidal Type III functional response and is therefore not consistent with the specialist Type II found in southern Portugal by Palma et al. [8]. ...
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Bonelli's eagle (Hieraaetus fasciatus), a threatened species in Western Europe has suffered a critical and severe decline in last two decade. In this paper, a qualitative analysis of an eco‐epidemiological model which consists of two prey and a predator is carried out. We proposed and designed a spatio‐temporal model to predict the distribution of a territorial predator, Bonelli's eagle and its two main prey species (rabbit and red‐legged partridge). Bounded positive solution, feasibility of the equilibria and their stability analysis are determined for the non‐spatial counterpart of the system. Criteria for diffusion‐driven instability caused by local random movements of rabbits, partridges and Bonelli's eagle are obtained. Possible implications of the result for Bonelli's eagle conservation are discussed. We show that the inclusion of second prey in the system can drastically change the dynamics from the single prey case. We also found that the presence of a second prey is beneficial for the conservation of the threatened Bonelli's eagle population in Europe. Results obtained from theoretical analysis of the non‐spatial model agree very well with the numerical simulation results. Lastly, via numerical simulation, we illustrate the effect of diffusion of the dynamical system in the spatial/spatiotemporal domain by different pattern formations.
... Optimal foraging theory suggests that predators select prey to maximize energy gain while minimizing energy expenditure (Stephens and Krebs 1986). Bonelli's eagles, though potentially facultative specialists (Glasser 1982; Moleón et al. 2007), likely rely on rabbits due to their (Columba palumbus), and red squirrels (Sciurus vulgaris), are present (Real 1991(Real , 1996Moleón et al. 2009), similar to other Mediterranean predators (Fedriani et al. 1998;Lozano et al. 2006). Greater habitat diversity in the most-used areas by eagles is higher in rabbit-poor territories, suggesting that eagles expend more effort and time foraging there. ...
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Understanding predator-prey interactions is important to determine the inter-relationships between species. Optimal foraging theory states that predators balance out energy expended with the energy gained from their prey. In the Iberian Peninsula, the European rabbit (Oryctolagus cuniculus) is a key prey species for endangered Bonelli’s eagle (Aquila fasciata). Thus, it is vital to understand how changes in rabbit abundance can influence habitat selection and territory use by Bonelli’s eagle. We studied 11 radio-tagged Bonelli’s eagles in their territories in Catalonia (NE Iberian Peninsula) and analysed the relationship between rabbit relative abundance, habitat selection and territory use of eagles. Rabbit relative abundance varied between territories, with shrublands hosting more rabbits, and eagles preferred shrublands and open areas for foraging and avoided dense forests. Spatial use by territorial eagles correlated positively with rabbit abundance in rabbit-rich territories, thereby supporting the idea that prey availability influences habitat selection. This result confirms optimal foraging strategies given that open habitats including shrublands tended to host more rabbits, thus providing better opportunities for prey detection and capture. Therefore, maintaining rabbit populations and their habitats (i.e., preserving open Mediterranean shrublands) would seem to be crucial for Bonelli’s eagle conservation. Our findings improve our understanding of predator-prey interactions and highlight the relationship between habitat structure, prey abundance and predator behaviour. In addition, our results emphasize the need for targeted conservation strategies designed to safeguard endangered species such as Bonelli’s eagle and maintain ecosystem integrity.
... In vertebrates, diet-specialist individuals are often more efficient or have greater fitness than generalists (Bolnick et al. 2002a, Svanb€ ack & Ekl€ ov 2003, Tinker et al. 2012; but see Woo et al. 2008, Whitfield et al. 2009), particularly when their preferred prey items are abundant (Terraube et al. 2011). Available evidence suggests that rabbits and partridges are key prey species for Bonelli's Eagle in southern Europe (Mole on et al. 2007(Mole on et al. , 2012a; however, since the outbreak of haemorrhagic disease in rabbits that has caused a drastic fall in this prey species' populations in recent decades in western Europe, Bonelli's Eagles have shifted their diets and have begun to consume fewer rabbits and increase their diet diversity (Real 1991, Mole on et al. 2009, 2012b, Resano-Mayor et al. 2014b, 2016. Furthermore, diets with little diversity, due to high consumption of either rabbits or pigeons, were correlated to higher productivity at the territory level (Resano-Mayor et al. 2014b) and, in the case of rabbits, to higher growth rates at the population level (Resano-Mayor et al. 2016). ...
Article
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The recognition of individual variation has fundamental implications for ecological, evolutionary and biodiversity conservation. There is increasing theoretical interest in how spatial and temporal variation in the environment can create differences in the demographic contribution of individuals over space and time. However, empirical information about the characteristics of the environmental drivers of key vital rates and their spatiotemporal variation is still scarce. Here, we used data generated by a monitoring scheme (1990–2015) of a population of a long‐lived territorial avian predator, Bonelli's Eagle Aquila fasciata , which included estimations of individuals' diet through stable isotope analysis (2008–15), to evaluate whether temporal consistency in spatially structured dietary patterns affects key demographic parameters, namely productivity and survival, at territory scales. We found strong within‐population heterogeneity in survival and reproduction rates associated with Eagle territories, with territory average values ranging, respectively, from 0.58 to 1.00 and from 0 to 1.71 for the overall study period. Reproduction and survival were predictable over, respectively, 4‐ and 3‐year periods for the bulk of the population, which suggests that the environmental drivers of these vital rates changed at these temporal scales. Interestingly, the characteristics of and the temporal variation in the diets of territorial individuals during these periods were associated with their survival and reproduction. Based on these findings, we suggest that spatial and temporal variations in trophic scenarios potentially act as meaningful drivers of intrapopulation demographic heterogeneity.
... Όλα τα δείγματα που συλλέγονται από μία θέση σε μία επίσκεψη θα πρέπει αν αναλυθούν αμέσως να καταγράφεται η προέλευσή τους , ή να αποθηκεύονται χωριστά από άλλες συλλογές για αναλύσεις στο μέλλον. Με τον τρόπο αυτό, αναλύοντας κάθε συλλογή δειγμάτων μπορεί να διερευνηθεί η διαφοροποίηση της δίαιτας μεταξύ επικρατειών, υποπληθυσμών και οι εποχιακές της διακυμάνσεις (Moleon et al., 2007;Resano-Mayor et al., 2016;Clouet et al., 2017). Η σήμανση σε κάθε περίπτωση να γίνεται σε μικρή χάρτινη ετικέτα μέσα στη σακούλα του δείγματος και γραμμένη με μολύβι. ...
... At the population level, almost all the non-reptilian prey categories showed considerable increases in the non-breeding season diet. Of avian taxa, increased frequencies of waterbirds and thrushes and pigeons, have been found also in other eagle diet studies during winter, possibly as these taxa more abundant and flocking at this period [53,54]. A tendency to exploit a locally abundant food source was apparent in territory 12 where waterbirds (gulls) were taken at relatively high frequencies as the site is situated close to a large refuse dump (category artificial land). ...
Article
Full-text available
Golden Eagles are resident in Greece and known to feed mainly on tortoises when breeding. However, information on alternative prey is scarce, especially during the tortoise brumation, that roughly coincides with the eagles’ non-breeding season. We analyzed 827 prey items collected from 12 territories over five territory years and 84 records of eagles hunting or feeding behavior. Tortoises dominated the breeding season diet (71% of prey categories on average) and over half of all hunting/feeding observations. While no spatial structure was evident, habitat variables such as forest canopy cover were important associates in golden eagle diet seasonally. A significant seasonal pattern emerged in diet diversity, using a subset of six territories with at least 10 samples per season. Eagles shifted from a narrow, reptile- based breeding season diet dominated by tortoises to a broader non-breeding season diet, that included more carrion, mammals and birds. Breeding season specialization on ectothermic prey is a trait usually associated with migratory raptors in the Western Palearctic. The observed dietary diversity expansion accompanied by residency in the absence of ectothermic prey, highlights the adaptability of the golden eagle, a generalist predator. Tortoise populations in Greece are of conservation concern and land use changes as well as climate change, such as development and land abandonment may increase the prevalence of catastrophic megafires, exacerbating the threats to the golden eagle’s main prey when breeding. We discuss this and other diet related conservation implications for the species in northern Greece.
... RHDV2; Camarda et al. 2014) and a loss of suitable habitat (Lo Valvo et al. 2017). Similar reductions in wild rabbit availability are likely to be affecting Bonelli's eagle in other European regions such as the Iberian Peninsula (Villafuerte et al. 1995;Moleón et al. 2007;Caro et al. 2011;Resano-Mayor et al. 2014). In particular, this raptor may select alternative prey species (pigeons and other birds), particularly in areas where rabbit haemorrhagic disease has drastically reduced rabbit abundances (Moleón et al. 2009(Moleón et al. , 2012Caro et al. 2011;Resano-Mayor et al. 2014). ...
Article
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Context Dietary analyses are essential to achieve a better understanding of animal ecology. In the case of endangered species, assessing dietary requirements is crucial to improve their management and conservation. The Bonelli’s eagle (Aquila fasciata) has experienced a severe decline throughout its breeding range in Europe and, in Italy, fewer than 50 pairs remain, and only in Sicily. This species is subject to major threats, including changes in landscape composition and, consequently, prey availability, which is further aggravated by the occurrence of viral diseases in the case of rabbits. Aims To provide current data on the diet of the Bonelli’s eagle in Sicily during the breeding period and to examine dietary shifts with regard to previous studies conducted in the same study area. To discuss possible implications for conservation of the Italian population of this endangered species. Methods We used a combination of three methods, including pellet analysis, collection of prey remains, and imagery from camera-traps installed at nests, to examine the diet of 12 breeding pairs of Bonelli’s eagle from 2011 to 2017. We compared this information with data collected between 1993 and 1998 in the same study area. Key results In number, birds were the most frequently predated items (61.6%), followed by mammals (36.88%) and reptiles (1.52%). However, in terms of biomass, mammals were the main prey (65.71%), followed by birds (34.12%) and reptiles (0.17%). There was a decrease over the course of the current decade in the consumption of European wild rabbit (Oryctolagus cuniculus), which was compensated for with an increase in both dietary diversity and breadth in bird consumption, a trend not observed in the earlier study in the same region. Conclusions Here, we provide an updated assessment of diet composition of Bonelli’s eagle during the breeding period. Interestingly, we found significant differences within the study period (2011–2017) in terms of frequency of occurrence, percentage of biomass, dietary diversity and dietary breadth in a species at risk. Furthermore, we found significant differences between the two study periods in both frequency and percentage of biomass, with significant changes in the consumption of lagomorphs and birds. Implications Our results indicated that shifts in the diet are linked to changes in prey abundance, which may be contributing to population declines in the Bonelli’s eagle population in Sicily. Overall, measures aimed at increasing main dietary prey should be promoted to favour occupation of new territories and enhance vital demographic parameters (i.e. breeding success and survival rate) of Bonelli’s eagle across the species range. This would be particularly important for small isolated populations such as the Sicilian one.
Article
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The influence of altitude on laying date was studied in a Bonelli's Eagle population breeding in south-eastern Spain (Granada province, a typical Mediterranean area). We dated 96 reproductions of 28 eagle pairs between 1994 and 1999. In addition, the influence of prey availability on laying date was studied in 1997. Laying date was calculated by means of the pterilar growth of the chicks (Appendix 1), considering an incubation period of 39 days. The altitude of the nesting sites was obtained from 1:25000 maps. Prey availability (rabbits Oryctolagus cuniculus and Red-legged Partridges Alectoris rufa) was estimated from census itineraries carried out for each nesting territory (8.2 km/territory on average) during March and April. The relationship between laying date and four independent variables (altitude, density of rabbits+Partridges, density of rabbits and density of Partridges) was tested by means of stepwise linear regressions. Laying date varied between 21 January and 17 March (mean: February 10; Fig. 1), a result that agreed with previous reports for Bonelli's Eagles in south-eastern Spain. There was a positive relationship between altitude and laying date when data from the six study years were considered together (Fig. 2), and this relationship was significant for three out of the six years when we tested the within-year correlations (Table 1). During 1997, altitude was the only variable selected by the regression analysis (R² = 0.395, P = 0.022, d.f. = 12; Fig. 3). There was a slight negative relationship between laying date and prey availability (partial correlation = -0.211), with a stronger association with rabbit availability (partial correlation = -0.228) than with Red-legged Partridge availablity (partial correlation = -0.121, Fig. 3). High prey availabilities as compared to other regions and/or microclimatic factors may be the causes of the marginal effect of trophic resources on the laying date of Bonelli´s Eagles in Granada.
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We studied factors influencing the length of the post-fledging period (from fledging to the start of dispersal) of Bonelli's Eagle (Hieraaetus fasciatus) nestlings in southwestern Spain, using 13 nestlings equipped with radiotransmitters. The age at fledging was negatively correlated with hatching date, but the duration of the post-fledging period was directly related to hatching date. This pattern could be explained by seasonal changes in prey abundance, especially that of wild rabbits (Oryctolagus cuniculus). Young increased their mobility throughout the post-fledging period, with a significant increase in the middle of the period. Dispersal began suddenly. The direction of dispersal was random, but most of the areas first used were located <25 km away and at lower altitudes than the nesting area.
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Birds form the main group of prey (48%), mainly species of pigeons Columba and red-legged partridge Alectoris rufa. Rabbits Oryctolagus cuniculus are the most important prey species (40%). There is high diveristy in the diet and eagles seem to exploit food resources within their breeding ranges efficiently. -from English summary
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The behaviour of five young Bonelli's Eagle Hieraaetus fasciatus was monitored from leaving the nest to dispersal in 1987 in three breeding sites in Catalonia (NE Spain), by means of radio-tracking techniques. On average, the young left the nests 63 days after hatching. The acquisition of flying ability was progressive, and was complete about 40-60 days after leaving the nest. The length of the dependence period (from leaving the nest to the end of parental food delivery) was 57-95 days. All the young suddenly left the natal area some days (11-36 days) after the last parental prey delivery was observed. The length of the post-nestling period (from leaving the nest to dispersal) was 77-113 days. Transition to independence involved a decline in parental investment. During the post-nestling period, some young were recorded up to 8.3 km away from the nest. A return of one young to its natal area three months after dispersal was also recorded.
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The factors limiting Bonelli's Eagle's nesting distribution in Granada (SE Spain) were studied. Habitat features of 74 cliffs were characterized, including cliffs with nests (35) and cliffs without nests (30), while considering that the species does not breed in areas where the January mean temperature is lower than 4°C. Abandoned cliffs (9) were also studied. For each cliff 18 variables were quantified evaluating habitat humanization, physiography, intraspecific competition with the Golden Eagle (Aquila chrysaetos), temperature and vegetation. The presence of Golden Eagle is an important limitation, especially in those areas with low cliff availability, althought the intolerant character of Golden Eagles to human presence favours Bonelli's Eagle in some areas. Bonelli's Eagle tolerates well habitat humanization, but there is a tendecy to avoid the most altered areas. The variables which best describe the breeding habitat were the distance to the nearest village and the topographical irregularity. Cultivations were a negative factor, unlike natural vegetation. There were not differences between ocupied cliffs and abandoned ones. :.
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Prey selection by five pairs of Bonelli's Eagles was studied during the breeding season in the Granada province, southeastern Spain, a typical Mediterranean area. The abundance of birds, squirrels and lizards was estimated by means of 1-3 line transects covering 3.5-9.5 km carried out within the territory of each eagle pair (Table 1). Rabbit abundance was estimated from counts of droppings carried out within 2-m wide bands along each transect. Rabbit density (No. individuals/ha) was then computed from dropping counts by means of a conversion factor developed from simultaneous counts of both droppings and rabbits carried out in one of the studied territories. Diet was characterised by means of pellet analyses (n = 51). Eleven prey categories were considered: rabbits, hares, squirrels, water voles Arvicola sapidus, Red-legged Partridges Alectoris rufa, pigeons, corvids, large birds, medium-sized birds, small birds and ocellated lizards Lacerta lepida. Prey selection was analysed by means of the Savage's index (Wi). The diet (n = 238 prey) was based on rabbits (39.9%) and, secondarily, on Red-legged Partridges (24.7%). Pigeons, ocellated lizards, corvids and other birds appeared in low proportions. Rabbits were positively selected by one out of the five pairs (Wi = 1.63, P < 0.01/2k; k = number of prey categories) and negatively selected by another one (Wi = 0.4, P < 0.005/2k). Red-legged Partridges were positively selected by one pair (Wi = 1.98, P < 0.005/2k) and pigeons were negatively selected by three pairs (Wi = 0.43, P < 0.05/2k; Wi = 0.15, P < 0.005/2k; Wi = 0.15, P < 0.005/2k). Small birds were consistently avoided by all pairs, whereas the remaining prey types were apparently captured on the basis of their availability. Bonelli's Eagles behaved as generalist predators except in the case of small birds, also showing a slight tendency to prefer Red-legged Partridges over other available prey. Rabbits and Red-legged Partridges seem to be the most profitable prey available in Mediterranean areas for Bonelli's Eagles, so that the low electivity of rabbits found was hardly expected. Daily patterns of habitat use by rabbits and the structure of the vegetation within eagle territories are discussed as likely causes for the differences in rabbit selectivity showed by the five studied pairs.