| Summary: | Black flies (Simuliidae) have been observed to harass Black Grouse (Tetrao tetrix) in northern Finland during summer. We studied experimentally whether group size affects the attack rate of black flies on Black Grouse. The increase in flock size diluted the rate of the black fly attacks in the Black Grouse, thereby demonstrating a clear encounter–dilu-tion effect. This study is – to our knowledge – the first in demonstrating an insect harass-ment dilution effect due to group living in an avian host, but our result are in agreement with previous studies carried out in various mammalian vertebrates. Joining other indi-viduals during summer when black flies are actively feeding should benefit Black Grouse. 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.
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