Food niche of Gyrfalcon Falco rusticolus nesting in the far north of Finland as compared with other choices of the species

Abstract

Food habits of the Gyrfalcon Falco rusticolus were studied in Finnish Forest Lapland by analyzing remnants of prey and pellets from six eyries . Birds made up 98-100% of the total prey biomass identified . Ptarmigan (Willow Grouse Lagopus lagopus and Rock Ptarmigan Lagopus mutus) were the main prey items, constituting 63-86% of the total biomass. In the main study area, SE Forest Lapland, the proportion of ptarmigan in the diet over six years was lower (49-71%) and the diversity higher (H'=0.49-0.65, H'max=0.90) than in Central or WForest Lapland. Large-sized tetraonids and waders were the main alternative prey in SE Forest Lapland, constituting 10-20% and 6-24% of the total biomass, respectively . The Gyrfalcon, a ptarmigan specialist, showed flexibility by responding to high densities of new prey items, e.g . microtines and by switching to preying on waders, especially the very conspicuous and adequate-sized Whimbrel Numenius phaeopus, when the vulnerability of the Willow Grouse decreased, although that species was still the preferred prey.