Increasing group size dilutes black fly attack rate in Black Grouse

Black flies (Simuliid ) have been observed to harass Black Grouse (Tetrao tetrix) innorthern Finland during summer. We studied experimentally whether group size affectsthe attack rate of black flies on Black Grouse. The increase in flock size diluted the rate ofthe black fly attacks in the Black Grouse, thereby demonstrating a clear encounter-dilutioneffect. This study is-to our knowledge-the first in demonstrating an insect harassmentdilution effect due to group living in an avian host, but our result are in agreementwith previous studies carried out in various mammalian vertebrates. Joining other individualsduring summer when black flies are actively feeding should benefit BlackGrouse. A reduction in insect harassment may at least partly explain why Black Grouse are often gregarious during summer, but does not exclude other reasons (e.g. a reduction in predation risk) for staying in groups.

Osmo Ratti, Ulriikka Ojanen & Pekka Helle, Ornis Fennica 83:86-90. 2006