Mass Changes of Least Auklets Aethia pusilla During the Breeding Season: Evidence for Programmed Loss of Mass

Patterns of seasonal variation in body mass by adult and sub-adult least auklets (Aethia pusilla, Pallas) were investigated at St Paul Island, Alaska using electronic balances to evaluate whether mass changes during the breeding season resulted from reproductive stress, or from a programmed response to increased flight demands during chick-rearing. Male and female auklets did not differ significantly either in mass or in the pattern of mass change. Sub-adults (2-year olds) were significantly lighter in mass than adults. Among adults, breeders that reached the chick provisioning stage were significantly heavier than failed breeders and non-breeders