| Summary: | Negative effects of late breeding on chick growth and creching and fledging ages of chicks have been reported for Chinstrap Penguins (Pygoscelis antarctica). In a Chinstrap Penguin rookery on Deception Island, we experimentally tested two hypotheses to explain these effects of hatching date: (1) late breeding pairs are formed by low-quality breeders that are not able to feed their chicks efficiently and are forced to leave them unguarded at younger ages; and (2) late breeding pairs experience a conflict between properly caring for their chicks and commencing the period of premolt reserve storage. By exchanging chicks among nests hatched six days apart, we separated the effects of quality of adults (as expressed by their breeding dates) from the hatching date of chicks. We measured bill and flipper length and weighed chicks at 17 and 44 days of age, and noted the age at which chicks were left unguarded by parents. Late-hatched chicks attained smaller sizes and masses and were left unguarded at earlier ages than early-hatched chicks independent of the breeding date of the adults raising them. Also, chicks hatched on the same date but raised by adults with different breeding dates reached the same sizes and masses and were left unguarded at similar ages. Thus, seasonal changes in chick growth and creching age are related to hatching date, not to differences in parental quality Our study was supported by grant ANT94-0036 from the Spanish C.I.C.Y.T. (Plan Nacional Antfirtico). Transport to and from Deception Island was made on the "Hesperides," Spanish Navy. Peer reviewed
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