| Summary: | Little penguins on Penguin Island, Western Australia, have a protracted breeding season lasting from May to November/December. Peak numbers of eggs are laid in June and September/October and some pairs may lay two clutches of eggs in one year. The birds moult in December/January and depending on the timing of their breeding, may still be rearing chicks as they fatten during the premoult period. Some birds may abandon chicks in order to moult. The coordination of metabolism, moulting and reproduction and co-ordination of all these with season, probably involves endocrine systems. We therefore measured the circulating concentrations of prolactin, thyroxine (T4) and triiodothyronine (T3) during the annual cycle of little penguins, using the birds from the Penguin Island colony and Perth Zoo. Prolactin concentrations did not appear to be strongly associated with annual changes in photoperiod but increased during incubation and chick rearing. There were no differences between the sexes, but captive penguins had higher values than those in the wild. Prolactin values decreased when the penguins began to lose their feathers during the moult cycle. Changes in the concentrations of T4 and T3 were not associated with stage of the breeding cycle but increases were observed during the moult. In particular, T4 appears to play a significant role in the stimulation of feather growth, with values during the premoult phase that were 3-fold higher than the yearly average. We conclude that, for little penguins, thyroid hormones and prolactin may play important roles in coordinating the physiological changes associated with breeding and feather moult. The lack of difference between the sexes reflects their similar roles in chick rearing, and the difference between captive and wild birds probably reflects the nature of the food supply in these two environments.
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