Biodiversity Data Journal :
Research Article
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Corresponding author: Dimitar Demerdzhiev (dimitar.demerdzhiev@gmail.com)
Academic editor: Caio J. Carlos
Received: 08 Nov 2021 | Accepted: 20 Dec 2021 | Published: 10 Jan 2022
© 2022 Dimitar Demerdzhiev, Zlatozar Boev, Dobromir Dobrev, Nikolay Terziev, Nedko Nedyalkov, Stoycho Stoychev, Tseno Petrov
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Demerdzhiev D, Boev Z, Dobrev D, Terziev N, Nedyalkov N, Stoychev S, Petrov T (2022) Diet of Eastern Imperial Eagle (Aquila heliaca) in Bulgaria: composition, distribution and variation. Biodiversity Data Journal 10: e77746. https://doi.org/10.3897/BDJ.10.e77746
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The Eastern Imperial Eagle (EIE) is a top predator exploiting different prey in different parts of its distribution. In this study, we summarise data collected over a long period of time (for 25 consecutive years), identifying key prey species in the different regions, as well as clarifying seasonal preferences in the eagle’s diet. Most studies on the EIE food composition covering different parts of the species distribution range analyse the breeding season, while data about the winter diet are scarce. To the best of our knowledge, this is the first study detailing the differences in EIE’s dietary preferences between the breeding and the winter periods. We identified 4891 specimens belonging to 196 different taxa, which represents the most comprehensive study considering the diet diversity of this threatened species. Mammals represented the largest proportion of the diet, followed by birds and reptiles. Northern White-breasted Hedgehog was the most common prey, accounting for 25.7% of the total prey caught and 26.75% of the biomass. The European Souslik was the second most important prey with 14.35% participation in the eagle’s diet, but with a 3.75% contribution to the biomass. As we predicted, prey composition and main prey species varied spatially and seasonally. Modelling differences in the EIE diet, we found that the “territory effect” had the strongest impact on the dietary variations. Diet diversity differed significantly between regions (F = 12.6, df = 4, p = 0.01). During the breeding season, eagles fed mainly on Hedgehogs (29.88%), Sousliks (16.85%) and Storks (7.74%), while the winter diet was predominantly small rodents (44.17%) and songbirds (13.96%). We found that top predators, such as EIE, have successfully adapted to a novel food source, which is abundant in the area. The detected flexibility in the diet of the species and its ability to switch to alternative prey, if available, when the primary prey decreased, should be considered when planning species conservation efforts. Investigating the temporal change of the main prey in the eagle’s diet is also crucial for further species conservation measures.
top predator, food spectrum, diet, raptors, long-term studies, population
The Eastern Imperial Eagle (Aquila heliaca), hereafter (EIE), is a large-size raptor species breeding from Central Europe, the Balkans, Central Asia and South Siberia to China and Mongolia (
The EIE is a top predator exploiting different prey in different parts of the distribution area (
Sparse data about the diet of the species in Bulgaria were published in the works of
In this study, we summarise data collected over 25 consecutive years, identifying the key prey species in the different regions, as well as clarifying the seasonal preferences in the eagle’s diet. Most of the studies on the EIE food composition covering different parts of the species distribution range analyse the breeding season (
We collected dietary data from eagles during the period 1996-2020 from 37 breeding sites distributed in six regions (Table
Distribution of localities and breeding attempts with collected food material.
Region |
Number of breeding sites |
Number of breeding attempts |
Sredna Gora Mnt. (SG) |
2 |
20 |
Eastern Rhodope Mnt. (ER) |
1 |
5 |
Sakar Mnt. |
10 |
126 |
Dervent Hights-Western foothills of Strandzha Mnt. (DHWstr.) |
7 |
65 |
Elhovo-Yambol Plain (EYP) |
10 |
58 |
Sliven Plain (SP) |
7 |
32 |
TOTAL |
37 |
306 |
Each nesting site was visited twice in each of the following periods: November-February, June-August (post-fledging period). Food remains, bones, feathers and pellets were collected inside and under nests and roosts (
The materials collected from 1 June to 31 August were referred to the eagles’ breeding season and those from 1 November to 1 March to their autumn-winter diet. The body mass of the specimens of the various species was determined by
In order to identify the diet differences between regions and seasons, the prey items were grouped into the following main categories, based on their specific ecological requirements: Lizards & Snakes (Squamata), Tortoises (Testudines), Water birds (Anatidae, Ardeidae), Poultry (Gallus gallus f. domestica, Anser anser f. domestica, Meleagris gallopavo f. domestica, Pavo cristatus f. domestica), Phasianids (Phasianidae), Gulls (Laridae), Doves (Columbidae, Feral Pigeon), Song birds (Non-Corvidae Passerines), Corvids (Corvidae), Stork (Ciconia ciconia), Raptors & Owls (Accipitridae, Falconidae, Strigidae, Tytonidae), Hedgehog, Hare, Souslik, Rodents (Rodentia excl. European Souslik), Carnivores (Carnivora), Carrion (Artiodactyla, Perissodactyla) and Other Animals.
To understand differences in diets amongst regions, we used Generalised Linear Mixed Models (GLMM) with Poisson distribution and Log link function. Our response variable was region and our predictors were food categories that showed high Likelihood Score (p ≤ 0.07) in the likelihood estimation (Table
Likelihood estimation of different food categories was used to describe the regional differences in the EIE diet. Categories included in GLMM’s are given in bold.
Variable |
Degrees of Freedom |
Likelihood Score |
p |
Lizards & Snakes |
1 |
4.6 |
0.0 3 |
Tortoises |
1 |
3.25 |
0.0 7 |
Poultry |
1 |
0.01 |
0.93 |
Stork |
1 |
2.43 |
0.12 |
Raptors & Owls |
1 |
1.85 |
0.17 |
Corvidae |
1 |
0.74 |
0.39 |
Other birds |
1 |
0.26 |
0.61 |
Hedgehog |
1 |
3.32 |
0.0 7 |
Hare |
1 |
2.35 |
0.12 |
Souslik |
1 |
1.26 |
0.26 |
Rodents |
1 |
3.4 |
0.07 |
Carnivores |
1 |
0.27 |
0.6 |
Other animals |
1 |
0.27 |
0.6 |
The relative importance of each model was estimated through the weight of AICc (w), so that all the weights for all models added up to 1. We also used explanatory parameter estimates with Lower (95%) and Upper CL (95%) and a probability value (p) of the explanatory factors.
To find out the diet differences between seasons, we used the non-parametric Mann-Whitney U Test with continuity correction.
All data were analysed using Statistica for Windows, Release 12 (
We identified 4891 specimens belonging to 196 different taxa (Suppl. material
Modelling differences in the EIE diet, we found that “random effect” had the strongest impact on the dietary variations (Table 3). This factor determined the first-ranked model both with regard to identified prey items (ΔAIC = 0.00, w = 0.46) and biomass contribution (ΔAIC = 0.00, w = 0.57). Presence of Northern White-breasted Hedgehog, Tortoises, Lizards & Snakes and biomass from Lizards & Snakes and Rodents shaped regional differences of eagle’s diet (Table 3). However, territories from Sakar Mnt. had a more powerful effect on dietary differences (β = 0.12 ± 0.05, p = 0.01) (Table
List of GLMMs used for the analysis of EIE diet; Food components in their participation as a prey item (A) and biomass contribution (B) were presented. All models with ∆AIC < 2 were considered best models; model weight value (w); RI – relative importance value of each of the candidate models; Parameter estimates ± SE, Lower (95%) and Upper CL (95%) of explanatory factors, their importance value (Wald Stat.) and a probability value (p) were taken from the average model.
N |
Model structure (A) |
AIC |
ΔAIC |
w |
RI |
p |
1 |
Random effect |
195.24 |
0.00 |
0.46 |
1 |
0.005 |
2 |
Hedgehog |
197.04 |
1.80 |
0.19 |
0.41 |
0.01 |
3 |
Tortoises |
197.11 |
1.87 |
0.18 |
0.39 |
0.01 |
4 |
Lizards & Snakes |
197.22 |
1.98 |
0.17 |
0.37 |
0.01 |
N |
Model structure (B) |
AIC |
ΔAIC |
w |
RI |
p |
1 |
Random effect |
195.24 |
0.00 |
0.57 |
1 |
0.005 |
2 |
Lizards & Snakes |
197.15 |
1.91 |
0.22 |
0.39 |
0.01 |
3 |
Rodents |
197.24 |
2.00 |
0.21 |
0.37 |
0.01 |
N |
Explanatory variables |
Estimate |
St. err. |
Wald Stat. |
Lower CL/Upper CL |
p |
1 |
Random effect |
4.72 |
0.11 |
1824.78 |
4.51 / 4.94 |
< 0.001 |
2 |
Hedgehog |
0.01 |
0.02 |
0.19 |
-0.02 / 0.04 |
0.66 |
3 |
Tortoises |
0.01 |
0.02 |
0.13 |
-0.03 / 0.04 |
0.72 |
4 |
Lizards & Snakes |
0.0005 |
0.03 |
0.0001 |
-0.06 / 0.06 |
0.99 |
5 |
Rodents |
-0.003 |
0.05 |
0.003 |
-0.1 / 0.1 |
0.96 |
Diet diversity differed significantly between regions (F = 12.6, df = 4, p = 0.01), being higher in the EYP (Hʹ = 3.488, n = 821) and the Sakar Mnt. (Hʹ = 3.119, n = 1961) and lower in the SG & ER (Hʹ = 2.516, n = 407). Regional differences were due to a much lower proportion of Lizards & Snakes in the SG (0.56%) and the SP (3.39%) and higher in the Sakar Mnt. (9.18%) and EYP (7.06%) (Fig.
The seasonal differences in the EIE diet were determined considering six major food components (Fig. 3). During the breeding season (n = 4096 preys), eagles fed mainly on Hedgehogs (29.88%), Sousliks (16.85%) and Storks (7.74%). Tortoises also showed significant seasonal differences (Z = 1.98, p = 0.05). The winter diet (n = 795 prey) included exclusively small rodents (44.17%) and songbirds (13.96%). The proportion of Carnivores was greater in the winter period (6.42%), although the differences when compared to the summer (2.39%) were not significant (Z = 1.88, p = 0.06). The consumption of carrion by eagles in winter (3.52%) was greater than in the breeding season (1.46%), but there was no trend (Z = 1.46, p = 0.14). The diet diversity index in winter (Hʹ = 3.474) was also larger than the mean value in the breeding season (Hʹ = 3.063) (Fig.
The great diversity of species in the food spectrum of the EIE proved its opportunism towards feeding. The identification of nearly two hundred different taxa of victims in our study supported the hypothesis of successful adaptation of the EIE to food sources (
Mammals were the most common group of vertebrates in the EIE’s diet in our study, as well as in previous studies (
Supporting previous findings for the EIE breeding in Kazakhstan (
Our study confirmed previous findings on the EIE diet in the SG (
As we predicted, the EIE used different food resources during different seasons. While Hedgehog, Souslik and White Stork were eaten in the breeding season, in winter, the eagles fed exclusively on different rodents, mostly voles (Microtus sp.). However, the share of songbirds also increased significantly in the winter diet. The EIE used different foraging techniques including active hunting, kleptoparasitism or followed and foraged after tractors in agricultural fields (
As with other studies (
Our study reveals that Northern White-breasted Hedgehog, European Souslik, White Stork and European Hare were the most important prey in the diet of the EIE in Bulgaria. In summary, the species has an extremely variable diet and our work provided clear evidence of the seasonal and spatial diet diversity concerning main food sources. Availability and abundance of different prey species in the individual eagle territory determine its foraging pattern and shape diet differences. Eagles occupied territories with high-density or abundant prey exploiting exclusively that particular prey and had lower diet diversity and vice versa and with no single prevailing food resource, the diet was more diverse.
We found that a top predator, such as EIE, had successfully adapted to the novel food source, which was abundant in the area. The detected flexibility in the diet of the species and its ability to switch to alternative prey, if available, when the primary prey decreased, should be considered when planning species conservation efforts. If the EIE utilises a single, most abundant prey source, it depends entirely on the availability of that specific prey source (
We would like to thank Ass. Prof. Georgi Popgeorgiev, Dr. Vladimir Vergilov, Ass. Prof. Nikolay Tzankov and Andrey Stoyanov for their assistance with the identification of the remains of amphibians and reptiles, to Prof. Vassil Popov and Prof. Nikolay Spassov for identification of some mammalian remains, to Prof. Mladen Jivkov and Ass. Prof. Tihomir Stefanov for identification of the fish prey. Our special thanks go to Atanas Demerdzhiev, Atanas Delchev, Aleksandar Georgiev, Dimitar Plachiyski, Georgi Georgiev, Georgi Gerdzhikov, Hristo Hristov, Iliya Iliev, Ivaylo Angelov, Kiril Metodiev, Krasimir Andonov, Krasimira Demerdzhieva, Mariana Valcheva, Marin Kurtev, Milan Bakalov, Stefan Avramov, Svetoslav Spasov, Vanyo Angelov, Vera Dyulgerska, Vladimir Dobrev, Vladimir Trifonov and Volen Arkumarev, who took part in the fieldwork. Without their assistance, this survey would not have been possible. We are grateful to Caio J. Carlos and Todd Katzner for their valuable review, comments and recommendations on the paper.
Life+ Project “Conservation of imperial eagle and saker falcon in key Natura 2000 sites in Bulgaria” (LIFE+07 NAT/BG/000068)
Life Project "Restoration and sustainable management of Imperial Eagle’s foraging habitats in key Natura 2000 sites in Bulgaria"(LIFE14 NAT/BG/001119
All ethical norms and standards were observed during the implementation of the surveys and the preparation of this manuscript.
Permits for exemption from the prohibitions related to the Eastern Imperial Eagle imposed by Art. 38 of the Biological Diversity Act, namely capturing for tagging, disturbance during the breeding season, rescuing individuals from abandoned broods and collecting found dead individuals, food material, as well as keeping and transporting the latter in order to implement relevant scientific surveys, were provided by the Ministry of Environment and Water of Bulgaria, No. 203/25.05.2009; No. 408/12.05.2011.
Dimitar Atanasov Demerdzhiev conceived and designed the study. Dimitar Atanasov Demerdzhiev, Dobromir Damyanov Dobrev, Nikolay Georgiev Terziev, Tseno Hristov Petrov, Nedko Petkov Nedyalkov and Stoycho Andonov Stoychev performed the fieldwork. Dimitar Atanasov Demerdzhiev, Zlatozar Nikolaev Boev, Nedko Petkov Nedyalkov and Dobromir Damyanov Dobrev processed and analysed the data. Dimitar Atanasov Demerdzhiev wrote the manuscript. All co-authors commented the manuscript. All co-authors read and approved the final manuscript.
The authors declare that they have no competing interests.