Abstract
In many bird species, eggs laid late in the laying period hatch after a shorter incubation period than early-laid eggs. However, the mechanisms that explain these seasonal declines in incubation periods among clutches remain poorly understood. In this study we investigated the plasticity of brood patch development during incubation in yellow-eyed penguins Megadyptes antipodes and established whether differences exist in brood patch formation among early, mean and late-breeding penguins. We also examined whether brood patch development was influenced by sex and age of birds. We then placed an artificial egg in nests a few days prior to egg laying to investigate whether the presence of an egg influences brood patch development and whether an advanced brood patch development at the time of egg laying causes declines in incubation periods. Initial brood patch width on the day the first egg was laid was dependent on sex and age, while the development of brood patch width after first egg laying was slower in early-laying birds than in mean- and late-laying birds. Initial brood patch temperature as well as temperature throughout incubation was largely dependent upon sex, whereby males had higher brood patch surface temperatures than females. Placement of an artificial egg in nests stimulated successfully brood patch development in manipulated birds, so that by the time they laid their own first egg, their brood patches were wider and had higher temperatures than those of control birds. Moreover, incubation periods of first eggs from manipulated nests were significantly shorter (43.5 days) than were those from control nests (47.3 days). Thus, variation in brood patch development and related differences in incubation temperature during early incubation could contribute to seasonal declines in incubation periods.