Benefits and costs of semi-colonial breeding in the Montagu's Harrier Circus pygargus

A total of 31 pairs of Montagu's Harrier Circus pygargus nesting on calcareous marshes near Che(3)m in eastern Poland were observed from 1991 to 1995 in ``Bagno Serebryskie'' nature reserve. This area has been included into Nature 2000 network as a special protection area for birds. Total time of observations was 1700 hours. The harriers were caught, ringed and individually marked by special colour wing tags. Observations each year started in April at the beginning of pair formation and continued until the start of post-fledging period in July. The size of a harrier nesting territory varied from 0.42ha to 2.25ha, averaging 1.14ha. Clumped territories were significantly smaller than solitary ones. The clumped territories were occupied earlier (as estimated from egg-laying dates) than solitary ones. Clutch size in clumped nests was larger than in solitary ones and the number of fledglings per nest was also higher in clumped territories. Semi-colonial nesting was safer in terms of predation risk. Copulation frequency was higher in semi-colonial than in solitary nests, suggesting a. higher risk for extra-pair copulations. Males from clumped nests spent more time inside territories than males from solitary nests. Most of observed copulations were preceded by food pass.