A change in clutch size in the Arctic Tern Sterna paradisaea in the northern Baltic

Abstract

The modal clutch size in Arctic Terns breeding on the southern coast of Finland was observed to differ from that reported in previous studies. In 1989 and 1990, the frequency of 3-egg clutches was 60% and 45%, respectively in one study colony, and 61 % and 54% in another colony. In 1990, the frequency of 3-egg clutches in a pooled sample of 65 nests was 54%. Spring was early in both years and breeding in terns was also early. It is possible that abundant food and early laying explain the switch in modal clutch size, but owing to a lack of data on fish abundance, a conclusive assessment cannot yet be made.