Pronounced Seasonal Diet Diversity Expansion of Golden Eagles (Aquila chrysaetos) in Northern Greece during the Non-Breeding Season: The Role of Tortoises

Lavrentis Sidiropoulos,D. Philip Whitfield,Dimitris Vasilakis,Christos Astaras,Haralambos Alivizatos,Vassiliki Kati

doi:10.3390/d14020135

Abstract

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.

Highlights

Food is one of the major limiting factors for raptor populations [1], affecting several population parameters
Generalist species can shift their diets towards a broader range during main prey decline periods, according to the Alternative Prey Hypothesis [8]
In Sweden, golden eagles displayed a plasticity on main prey depending on the habitat affecting its availability across its national distribution, and even high specialization locally [14,15]

Summary

Introduction

Food is one of the major limiting factors for raptor populations [1], affecting several population parameters. Food availability and diet metrics have been documented to affect densities [2], breeding performance [3,4], convergence of individuals during dispersal in food rich areas [5], nestling condition metrics [6] and long-term population persistence [7]. Several raptor species tend to consume a few, readily available and profitable taxa given the opportunity, as demonstrated by population level scale studies. The golden eagle is the most widespread Holarctic eagle, is highly adaptable in its diet as a generalist predator, despite often displaying specialism at low spatial and temporal scales. In Sweden, golden eagles displayed a plasticity on main prey depending on the habitat affecting its availability across its national distribution, and even high specialization locally [14,15]. I In Utah, USA, habitat variables explained best the occurrence of main prey types in golden Eagle diets [16]

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Conclusion