Roost site selection of Great Horned Owls in relation to black fly activity: An anti-parasite behavior?

We document a shift in roosting behavior of Great Homed Owls (Bubo virginia nus) from winter and late spring to summer. During summer, Great Homed Owls roosted near the ground or exposed on open ground, whereas they chose conc ealed perches at mid-canopy level for the rest of the year as typical for f orest owls. This shift of roosting behavior coincided with the emergence of ornithophilic black flies, which transmit avian malaria (Leucocytozoon spp .). The shift in roosting behavior was consistent with measurements of para site exposure at different habitat positions. Black Ay activity was highest at mid-canopy level, and almost no black flies were active on open ground. Ground-roosting was not caused by poorly developed Eying capability of juv eniles, because solitarily-roosting adult owls showed the same behavioral s hift in a second year of study. Black flies and avian malaria are widely di stributed, and the effect of the vertical distribution of these parasites i n forests on roosting, nesting, and foraging of sylvatic birds deserves fur ther study.