Relations Between Willow Ptarmigan (Lagopus Lagopus) Density and Gyrfalcon (Falco rusticolus ) Breeding Performance in Sweden.

ptarmigan is considered a key factor for successful breeding of Gyrfalcons (e.g., Cade et al. 1998, Hagen 1952, Koskimies 1999, Potapov and Sale 2005). In many parts of Europe, Lagopusspp. often represent over 90% of the Gyrfalcon’s diet during the breeding season (Koskimies and Sulkava 2002, Nielsen 2003, Nystrom et al. 2005). Willow Ptarmigan( Lagopus lagopus) is the most important prey for the Finnish Gyrfalcon population (Koskimies 2005) while Rock Ptarmigan ( Lagopusmuta) is the main prey in Iceland (Nielsen 2003), and an important part of the diet in the high alpine areas in Sweden and Norway (Nystrom et al. 2005). This study, focusing on the spatial and temporal relations between ptarmigan density and Gyrfalcon breeding performance, analyzed data from 55 Gyrfalcon territories and ten ptarmigan census areas in Central Sweden at 63°–64°N between 1996 and 2010.