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

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.

Predation impact on threatened spur-thighed tortoises by golden eagles when main prey is scarce

“…2). In other Palearctic areas where golden eagles and Testudo tortoises coexist, tortoise consumption by this raptor is more frequent (53-93%N) [24][25][26][27][28][29]48 . The presence in Mediterranean Spain of a key medium-sized prey species, namely the Eurasian rabbit 49 , may be behind the relatively low representation of tortoises in the golden eagle's diet in our study area.…”

“…In some areas, tortoises can represent up to c. 90% of the golden eagle’s diet 28 . This indicates that predation by golden eagles on tortoises is common wherever they coexist in the Western Palearctic 29 . Although some studies have described the role of tortoises in eagles’ diet, the potential effects on tortoise populations have not been previously evaluated.…”

Habitat Impacts on the Golden Eagle’s Foraging Ecology and Nest Site Selection in Poland

“…The minimum number of individuals (MNI) at each collection's lowest possible taxonomic level was calculated from distinctive anatomical features by taking the minimum number derived from each source and combining them. If we found, e.g., the bill and feathers of the Ural Owl Strix uralensis, we counted them as only one prey [21]. For the identified prey species, the habitat in which the GE hunted them was assigned.…”

“…Ungulate calves have been important prey in the Italian Alps [43] and the tundra region [16,17]. Hares and other Leporidae were the vital prey of the GE in Finland [16], Belarussian Polessya [41], the Pyrenees [19], Great Britain [12,18], the mountain tundra [17], as well as in the western United States [21]. Almost throughout its range, the GE often captured larger rodents [12,16,17,19].…”

“…The basis of its food is birds and mammals, and seldom reptilians, consumed in varying proportions. Numerous studies of food composition have documented the GE's strong ecological plasticity and ability to successfully prey on a variety of species, usually at a biomass from 0.5 to 3 kg, e.g., [16][17][18][19][20][21]. Diet composition shows a high variability depending on the availability of suitable prey in local conditions.…”