Breeding biology of the Herring Gull Larus argentatus in the northern Baltic

Abstract

Study of the breeding biology of a small Herring Gull colony off Hanko, southern Finland in five seasons, 1978 and 1983-86, showed that the gulls occupy the colony in mid-March, arriving in the area a few weeks earlier. Non-breeding cohorts arrive in April and laying begins in the second half of April. Median dates for laying ranged from 25 April to 4May. Laying was related to the ice break-up. In one season early breeders lost eggs due to cold weather. Clutch-size was close to three, and only 11% of the full clutches had two eggs. Clutch size decreased with season, but variation was pronounced. Full clutches of three eggs were ofequal volume in all years, averaging about 28% of female weight. Only 5 ±4.5% of all eggs laid were lost during incubation . Total losses up to the time of hatching (average 13 ± 10%) varied significantly between years. Ofthe hatched chicks, 29 ± 13% died, mostly during their first week oflife . On average, only 6 ± 5% of those surviving the first week died before fledging . Of the hatched chicks, 63 ± 12% fledged successfully, the mean number of young fledging per pair being 1.55 ± 0.29. Chicks left the colony in early August. Compared with other NW European populations, Herring Gulls offHanko breed early, and their breeding success is high.