Golden Eagles on the Swedish mountain tundra – diet and breeding success in relation to prey fluctuations

Abstract

We studied the diet and the relationship between prey density fluctuations and breeding success of a Golden Eagle (Aquila chrysaetos) population on the mountain tundra region of northern Sweden. We used a new PCR based method to analyse the DNA in bone frag-ments from Golden Eagle prey remains. This allowed us to accurately identify the Ptarmi-gan species that the bone fragments originated from, and hence, establish the proportions of Ptarmigan species in the eagle's diet. We could conclude that Ptarmigan species (Lagopus spp.) are the most important prey category for this Golden Eagle population (63% of all identified prey), and that Willow Ptarmigan (L. lagopus) occurred more fre-quently in the diet than Rock Ptarmigan (L. mutus) did (Willow Ptarmigan 38%, Rock Ptarmigan 25%). Other important prey included reindeer (Rangifer tarandus), mountain hare (Lepus timidus) and microtine rodents. The Golden Eagles managed to maintain a relatively broad food niche, despite an environment with low prey diversity. Microtine ro-dents, hare and Ptarmigan populations showed similar population fluctuations in the study area. The breeding success of the Golden Eagles showed a strong relationship to the yearly density index of the most important prey category, the Ptarmigan species.