Abstract
Breeding success in Montagu’s Harrier (Circus pygargus) was studied in peat bogs of eastern Poland during the period 1996–2002. Harriers nested only in the area covered with great fen sedge (Cladium mariscus). The studied Harriers laid from 1 to 6 eggs, on average 4.09 eggs (n = 118). With the progression of the season, no significant decline in clutch size was recorded. The highest values of clutch size were found for pairs nesting in five nest aggregations. Harriers fledged on average 2.49 young per nest and 1.46 young per breeding pair. Early clutches in semicolonies were larger than later ones. Aggregations of five and four nests were found optimal in terms of clutch size, total loss and fledging success. The results of the study agree with the hypothesis that reproductive success may be highest at intermediate colony size.
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