MASS CHANGES OF LEAST AUKLETS AETHIA-PUSILLA DURING THE BREEDING-SEASON - EVIDENCE FOR PROGRAMMED LOSS OF MASS

1. 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. 2. Male and female auklets did not differ significantly either in mass or in the pattern of mass change. 3. 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. 4. Overall, the pattern of mass variation during the breeding season was nonlinear, with mass increasing during the prelaying period, stable or increasing during incubation, and declining somewhat by chick rearing. 5. Breeding adult least auklets experienced an abrupt mass loss of 6 g (7% of mean body mass) in 1988 and 3 g (4% of body mass) in 1989 that coincided with the hatching of their chick, afterwards mass stabilized. These mass losses were predicted to reduce flight costs by about 10% (1988) and 4% (1989). 6. The brooding period was not associated with abruptly increased energetic expenditure compared with incubation, because the brooding parents spent less time in the nesting crevice and alternated time at the nest more frequently than during incubation. 7. These patterns are consistent with the hypothesis that mass loss is a programmed response to increasing flight demands, rather than a consequence of reproductive stress.