Population-Changes, Breeding Cycles and Breeding Success Over Six Years in a Seabird Colony at Michaelmas Cay, Queensland

Abstract

Seabirds were counted on Michaelmas Cay every month from May 1984 to April 1990. Seven species bred during the study. The major breeding species were Sooty Tern Sterna fuscata, Common Noddy Anous stolidus, Crested Tern S. bergii and Lesser Crested Tern S. bengalensis. Silver Gull Larus novaehollandiae, Black-naped Tern S. sumatrana and Bridled Tern S. anathaetus bred in some years in small numbers. Sooty Terns had a sub-annual breeding cycle of 8.6 months duration, measured between peaks of breeding numbers, while the other species were annual breeders, five in summer and one (Silver Gull) in winter. In successive breeding cycles, the sizes of breeding populations of Sooty Tern and Common Noddy both fluctuated strongly. Crested Tern numbers increased slightly then decreased, and Lesser Crested Tern numbers increased strongly then decreased again. Average Silver Gull numbers remained fairly stable over the study period. Fledging success varied considerably among breeding cycles of Sooty Tern, Common Noddy and Crested Tern. Lesser Crested Terns bred successfully in three cycles and showed increasing fledging success over the study period. Climate had a major influence on breeding populations of Sooty Tern and Common Noddy. Three cyclones were recorded; one altered the periodicity of Sooty Tern breeding cycles and two affected its breeding numbers and success. In mid-year, prolonged periods of strong winds caused breeding failures in Sooty Tern and Common Noddy. Erosion of the cay also caused nest losses. Human disturbance had little effect on breeding populations.