Relationships between foraging behaviour, diet and reproductive success at an urban colony of little penguins (Eudyptula minor), St Kilda, Australia

Little penguins have shown an overall pattern of decline throughout their distribution, thus it was considered unusual for a new colony to establish themselves recently near the heart of a city at St Kilda, Victoria, Australia. Since they were first observed in the 1970’s, this colony has grown from a few breeding pairs to ~1000 individuals. It is the factors that have contributed to this population success that this study aims to examine, specifically the relationships between colony’s foraging behaviour, diet and subsequent reproductive success. Penguin foraging behaviour was studied during three breeding seasons by satellite transmitters, global positioning system (GPS) loggers and time-depth recorders (TDR). Results revealed that the penguins have a small foraging area ~1500 ha in the north of Port Phillip bay. Maximum distance travelled from the colony each day is ~13 kms and total horizontal distance covered is ~34 km, less than that cited for other penguin colonies. Diving was conducted mostly to depths ≤10 m, with ~800 dives performed in a day, comparable to other little penguin colonies. Use of the combined GPS and TDR revealed that the penguins spent a disproportionately high amount of time foraging in the artificially dredged shipping channels that occur within their foraging area. This foraging study also found that the combined loggers providing more accurate and useful data at the small scale than separately deployed satellite transmitters and TDRs. Diet at the penguin colony was monitored continuously over a two year period using stomach content and stable isotope analysis. Stable isotopes were better able to detect seasonal variation in diet, while stomach contents were useful for determining dietary diversity and age cohorts of the prey consumed. The diet was dominated by Australian anchovy (Engraulis australis) and southern garfish (Hypohamphus melanochir). Australian anchovies were common during the pre-breeding and early stages of breeding throughout spring and early summer. Southern garfish increased in the diet from chick-rearing through the non-breeding season. Adult penguins switched from Australian anchovy to southern garfish to feed their chicks, presumably due to the higher energetic, protein and fat content of this species. Reproductive success determined by the number of chicks fledged per pair and peak chick weights was similarly low in all years of this study. The mean egg-laying date was earlier in 2008 than other years, and the reproductive success was also greatest in this year, being considered ‘average’ for little penguins. Double brooding was highest in 2006, but this was also the year with the latest mean egg-laying date and the lowest fledging rate from first clutches. Successful breeding was associated with quality prey rather than increased double brooding. St Kilda little penguin colony is an example of a native animal population that has successfully exploited changes in both the terrestrial (breakwater) and marine (shipping channel) environments. Prey availability in close proximity to their colony is essential for the continuation of this population; their survival in the urban environment can be supported through the careful management of their prey stocks.