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Studying the demographic drivers of an increasing Imperial Eagle population to inform conservation management

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Abstract

Assessing whether conservation management actions are effective requires a good understanding of the demographic parameters that contribute to the population growth rate. Among the key demographic parameters influencing a population, immigration is one of the most difficult to measure empirically but may mask or accentuate the effects of conservation measures. We use an integrated population model to assess whether a population increase of a large raptor species can be explained by high fecundity and survival resulting from local conservation measures, or whether immigration may have contributed to population growth. We monitored the Eastern Imperial Eagle (Aquila heliaca) population in Bulgaria from 1998 to 2014, and tracked juveniles using satellite transmitters to estimate survival probability over the first three years of life. We used intensive territory monitoring of breeding birds to estimate survival probabilities and fecundity of birds older than 3 years. The Imperial Eagle population in Bulgaria increased by about 11 % per year between 1998 and 2014 (λ = 1.111, 95 % credible interval 1.076–1.156). While local conservation measures have succeeded in reducing nest loss and the mortality of adults (adult survival = 0.924; 0.887–0.955), high mortality of juveniles during their first year of life resulted in only 12 % of fledglings surviving to adult age. Based on these survival probabilities and the estimated fecundity of breeding pairs in Bulgaria (1.063; 0.932–1.203), some immigration may have contributed to the population growth in Bulgaria. Because the integrated population model accounts for all the uncertainty associated with disparate data sources, the estimated immigration rate was too imprecise (0.143–2.862 young birds per breeding pair) to quantify the relative importance of immigration. Future conservation measures for Imperial Eagles need to focus on improving the survival of juvenile birds, particularly the reduction of electrocution risk in Eastern Europe and the Middle East.

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Acknowledgments

Many BSPB/BirdLife volunteers assisted in monitoring and conservation efforts, and we are grateful for the assistance by Ivaylo Angelov, Petar Iankov, Hristo Hristov, Nikolai Terziev, Tzeno Petrov, Georgi Popgeorgiev, Prof. Zlatozar Boev, Vladimir Trifonov, Nedko Nedyalkov, Atanas Demerdzhiev, Krasimira Demerdzhieva, Georgi Georgiev, Dimitar Plachiyski, Georgi Gerdzhikov, Vladimir Dobrev, Ivayla Klimentova, Vera Dyulgerska, Kiril Metodiev, Vanyo Angelov, Petya Karpuzova, Yanko Yankov, Aleksandar Georgiev, Vasilena Chobanova, Girgina Daskalova, Marin Kurtev, Valentin Velev, Tatiana Veleva, Georgi Manolov, Nikolay Dolapchiev, Petya Kurteva, Volen Arkumarev, Ivan Lafchiev, Dimitar Aygatov, Kiril Mihalev, Emil Yordanov. We are grateful to Gradimir Gradev, Ivaylo Klisurov, Simeon Marin and all the colleagues from Green Balkans for sharing information and collaboration in implementing conservation activities. Márton Horváth, András Kovács, Tamás Szitta, Gabor Papp and Magyar Madártani és Természetvédelmi Egyesület (BirdLife in Hungary) shared their expertise on raptor ecology, tree climbing, and preliminary results of genetic analyses. Dimitris Vasilakis from WWF Greece and Richard Cuthbert and Guy Anderson from the RSPB (Birdlife in UK), and Bernd-Ulrich Meyburg assisted with satellite tagging. Fitsum Abadi shared valuable thoughts on the integrated population model and we appreciate his help. Miguel Ferrer and an anonymous reviewer provided valuable comments on a previous draft of the manuscript. This work was funded by the Bulgarian Ministry of Environment and Waters, by the Bulgaria-Turkey cross-border cooperation programme under the project “Strandzha-Sakar—the realm of the eagles”, and by the LIFE + Program of the European Union under the project „Conservation of Imperial Eagle and Saker Falcon in key Natura 2000 sites in Bulgaria” LIFE07 NAT/BG/000068.

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Authors and Affiliations

  1. Bulgarian Society for the Protection of Birds/BSPB, BirdLife Bulgaria, Yavorov complex, bl. 71, ent. 4, app. 1, Sofia, Bulgaria

    Dimitar Demerdzhiev, Stoycho Stoychev, Dobromir Dobrev & Svetoslav Spasov

  2. RSPB Centre for Conservation Science, Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire, SG19 2DL, UK

    Steffen Oppel

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  1. Dimitar Demerdzhiev
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  2. Stoycho Stoychev
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  3. Dobromir Dobrev
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  4. Svetoslav Spasov
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  5. Steffen Oppel
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Correspondence to Dimitar Demerdzhiev.

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Communicated by Stephen Garnett.

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Demerdzhiev, D., Stoychev, S., Dobrev, D. et al. Studying the demographic drivers of an increasing Imperial Eagle population to inform conservation management. Biodivers Conserv 24, 627–639 (2015). https://doi.org/10.1007/s10531-014-0841-0

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