Reproductive Periodicity in a Population of Crested Terns, Sterna bergii Lichtenstein in South Australia

Abstract

Crested terns breeding in the Fremantle area of south-western Australia nest over an 8-month period, from early April to early November. However, laying is markedly bimodal, with subseasons in autumn and spring. Observations of individually marked breeding crested terns indicated a sedentary population utilizing a number of alternative, traditional colony sites all of which were within the study area. Individual crested terns tended to lay in the same part of the season as in the previous year. Colonies made up of terns with similarly phased reproductive cycles formed at different times within the protracted laying period. Individuals completed only one nesting cycle within a season, remaining in breeding condition for approximately 11-13 weeks. Breeding cycles were broadly circannual but potentially sub-annual. Despite superficial similarities at the populational level, this laying pattern differs markedly from that recorded for the silver gull in the same area. The pattern of breeding shown by crested terns therefore provides a second model to explain the double-nesting phenomenon observed in a number of seabird species on the western Australian coastline.