Seasonal Changes in Food Habits of Gyrfalcons in NE-Iceland

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Abstract
Food habits of three subpopulations of the Gyrfalcon Falco rusticolus were studied in coastal, lakeland and heathland habitats of NE-Iceland by analysis of 2,316 pellets collected from February 1984-July 1985. No coastal and lakeland Gyrfalcons traveled more than 7.5 km to reach areas with plenty of waterfowl or seabirds (average 3.4 km in winter). Heathland falcons traveled farther to reach equally rich areas for these kinds of prey, e.g. in winter 13-33 km (average 12.9 km). Rock Ptarmigan Lagopus mutus was the main prey for all Gyrfalcons constituting 61% of total pellet volume. Rock Ptarmigan was most important during the early phase of the breeding season (April-June) and in autumn and early winter (August-December). Factors determining high vulnerability in spring seem to be behaviour and coloration of males and in later summer, the large number of naive young. Importance of Rock Ptarmigan on heathland territories as measured by pellet volume ranged from 61-100% for the different collection periods, and 8-85% and 13-91% on coastal and lakeland territores respectively. Other prey groups of importance on heathland territories were waders (max. 12%) in summer and wood mouse Apodemus sylvaticus (max. 20%) and waterfowl (max. 20%) in winter. Lakeland Gyrfalcons increased their take of waterfowl (max. 56%) and waders (max. 13%)in summer and passerines (max. 19%) and wood mice (max. 43%) in winter. Coastal Gyrfalcons had the most variable diet; they increased their take of waterfowl (max. 42%) and waders (max. 21%) in summer and waterfowl (max. 68%) and alcids (max. 38%) in winter. Switching from Rock Ptarmigan to alternate preyinvolved in most cases a change of hunting habitat. from upland areas to wetland or coastal areas.