Effects of food variability on growth rates, fledging sizes and reproductive success in the Yellow‐eyed Penguin Megadyptes antipodes

Effects of a change of diet on growth rates and fledging sizes of Yellow‐eyed Penguins Megadyptes antipodes were examined at two breeding areas on South Island, New Zealand, during two breeding seasons. An adverse change in diet was observed in the second season. Evidence for this included depressed growth rates of weight, differential growth of weight and most morphometric parameters between one‐ and two‐chick nests in the second season, lower fledging weights, lower adult body weights, delayed moult, higher chick mortality and higher adult mortality during moult. The change in diet is suggested as being from one including oil‐rich prey species, to one of oil‐poor species. Growth rates of first‐ and second‐hatched chicks, and of survivors and non‐survivors within a brood were not significantly different in either season, and growth rates of two‐chick broods were only slightly slower than one‐chick broods for some parameters in the second season. This, and synchronous hatching of chicks, equal egg‐size and lack of sibling competition during feeding sessions, suggests that brood reduction is not an option available to Yellow‐eyed Penguins, and that food supply may not be a limiting factor in the majority of breeding seasons. Few changes in growth rates of morphometric parameters at either breeding area, and similar absolute sizes at fledging, indicate that slowing of growth rates of morphometric parameters only occurs when feeding conditions are so bad as to result in mortality and that, although fledging periods may be longer, patterns of development remain essentially unchanged.