FOOD NICHE OF THE GYRFALCON FALCO-RUSTICOLUS NESTING IN THE FAR NORTHOF FINLAND AS COMPARED WITH OTHER CHOICES OF THE SPECIES

Food habits of the Gyrfalcon Falco rusticolus were studied in Finnish Forest Lapland by analyzing remnants of prey and pellets from six eyri es. Birds made up 98-100% of the total prey biomass identified. Ptarmi gan (Willow Grouse Lagopus lagopus and Rock Ptarmigan Lagopus mutus) w ere the main prey items, constituting 63-86% df 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 W Forest Lapland. Large -sized tetraonids and waders were the main alternative prey in SE Fore st Lapland, constituting 10-20% and 6-24% of the total biomass, respec tively. The Gyrfalcon, a ptarmigan specialist, showed flexibility by r esponding to high densities of new prey items, e.g. microtines and by switching to preying on waders, especially the very conspicuous and ad equate-sized Whimbrel Numenius phaeopus, when the vulnerability of the Willow Grouse decreased, although that species was still the preferre d prey.