Effect of local prey availability on gyrfalcon diet: DNA analysis on ptarmigan remains at nest sites

Abstract

AbstractThe aim of this study was to investigate how the diet of gyrfalcon Falco rusticolus in northern Sweden was affected by the relative availability of its two main prey species: rock ptarmigan Lagopus mutus and willow ptarmigan Lagopus lagopus. In order to do so, we needed a method to estimate the gyrfalcon's diet proportions of rock and willow ptarmigan from prey remains that we collected from nest sites in separate breeding territories. We also needed a method to calculate the availability of the two prey species in the same breeding territories that the prey remains originated from. We could then compare the diet proportions with prey availability and investigate if the gyrfalcons utilized the two species strictly in relation to their densities, or if they showed a preference for any of the prey species. Morphometric identification to species level from ptarmigan remains was not possible. Therefore, we developed a PCR‐based process of DNA analysis, which could be applied on any ptarmigan bone or bone remains. This method allowed us to establish the ratio of rock and willow ptarmigan in gyrfalcon diets that originated from single gyrfalcon breeding occasions. The relative availability of the two ptarmigan species in gyrfalcon breeding territories was calculated with a GIS model that incorporated observations on ptarmigan habitat preferences. The DNA identification was performed on 176 ptarmigan bones from 13 different breeding occasions occurring in five different territories. The results indicated that the two ptarmigan species comprised at least 93% of the average gyrfalcon diet, and that rock ptarmigan was the most common prey during all 13 breeding occasions. There was a positive relationship between the relative amount of rock ptarmigan in the diet and the proportion of rock ptarmigan habitat in the territories; hence, the gyrfalcons ptarmigan utilization seemed to be density dependent. However, rock ptarmigan was found to be overrepresented in the diet, which may reflect a preference for rock ptarmigan over willow ptarmigan. The conservation implications of these findings in relation to ptarmigan hunting are discussed.