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Finch Flock Size and Risk of Hawk Predation at a Migratory Stopover Site
Vol. 106, No. 2 (Apr., 1989), pp. 225-232
Published by: American Ornithological Society
Stable URL: http://www.jstor.org/stable/4087716
Page Count: 8
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I studied migratory flocks of finches (Fringilla coelebs and F. montifringilla) feeding during three autumn and two spring seasons at a stopover site in South Sweden. Raptors attacked flocks repeatedly, and the finches showed obvious adaptations to the risk. A total of 304 attacks were recorded, of which 270 were from European Sparrowhawks (Accipiter nisus). Attack frequency was significantly higher in autumn (1.9 attacks/h) than in spring (0.3 attacks/h). In autumn, finch flock size varied from 10 to 10,000. Both attack frequency and hunting success increased with flock size. The risk for an individual finch to be killed (successful attacks per hour and individual) showed no correlation with flock size. Thus predation seemed not to be the primary cause for formation of large finch flocks. Instead, patchy distribution of the preferred food presumably causes large numbers of finches to aggregate. Once aggregated at a field, the best antipredator strategy for the finches is probably to crowd into a dense flock with synchronized feeding. I estimated that ca. 10% of the finches were killed by raptors during the autumn migration.
The Auk © 1989 American Ornithologists' Union