The diet of the Marsh Harrier Circus aeruginosus breeding on the isle of Hailuoto compared to other raptors in northern Finland

Abstract

The Marsh Harrier Circus aeruginosus population has expanded rapidly during the last decades in northern Finland. We studied the diet of Marsh Harrier during the breeding seasons 2002–2009 on the isle of Hailuoto in the Gulf of Bothnia in Finland. We com-pared these results to those of other raptors in the region, and attempted to relate potential differences in the diet composition to population trends. Birds accounted for 73–93% of prey numbers in the Marsh Harrier during the breeding season. Passerine birds were the most numerous during the nestling (63%) and fledging periods (21%), while ducks and gulls/terns had the highest biomass (29% and 12%, respectively). Muskrat (Ondatra zibethica) was the most important prey among mammals by number and weight (15% and 18%, respectively). Compositional analysis showed that the Marsh Harrier depredated water and shore birds opportunistically. Based on multidimensional scaling, the food niche of the Marsh Harrier was distinctive from those of five other raptor species. Analy-sis of diet breadth and annual change in population size suggested that Marsh Harrier diet breadth was wider and its annual population increase among the fastest, compared to other raptor species. Annual population change of raptors was positive among species that were rare in the beginning of monitoring, i.e., in the early 1970s, but was negative among species that were common at that time. No significant relationships were observed between diet breadth and population growth among the studied species.