Responses of Herring Gulls Larus argentatus and Common Gulls L. canus to warm years: early migration and early breeding

Abstrakti

Monitoring of two breeding colonies in 1983-1991 indicated that both Herring Gulls and Common Gulls altered their migration schedule and time of breeding in very early springs (1989 to 1991). The date of the ice break-up dates varied during the study period, the maximum difference between dates being 96 days. There was a trend towards an earlier break-up in 1983-1991, and the break-up dates correlated with the maximum ice-cover in the Baltic, reflecting the general severity of the winter . The Herring Gulls occupied the breeding colony up to 54 days earlier in the earliest spring (1990) than in late springs, the Common Gulls 26 days earlier. Early arrival was followed by early breeding. The maximum difference between years in the initiation of the first nest was 23 days in the Herring Gull and 11 days in the Common Gull. The response to a warm year is more pronounced with regard to the timing ofmigration and colony occupancy, than with regard to the timing of laying . As only one group of breeding birds was monitored for each species, the data suggest that individual adults are able to alter their migration behaviour from year to year . The Herring Gulls were behaviourally more plastic than the Common Gulls. This may be due to the wintering strategy ; Herring Gulls stay chiefly within the Baltic and are probably able to monitor the meteorological conditions more readily than Common Gulls which migrate outside the Baltic area.