Linking dietary shifts and reproductive failure in seabirds: a stable isotope approach

Summary Diet‐related breeding failure in seabirds has been attributed to declines in key prey abundance, the quality of prey and overall prey availability. However, identifying which aspect of diet is responsible for reproductive failure is challenging due to the practicalities of measuring prey utilization and the actual availability and abundance of those resources. In this study, stable isotope‐based Bayesian models, in combination with indices of resource availability, were used to assess the links between prey availability, seabird diet and reproductive success in a generalist, inshore top predator, the little penguin, Eudyptula minor . The most probable causes for the sharp decrease in little penguin reproductive performance were diminished localized populations of anchovies, Engraulis australis , in combination with the scarcity of alternative prey. Low dietary diversity and the consumption of low trophic value prey were observed in this period. In the contrasting following year, penguins consumed increased levels of anchovy as well as a high diversity of prey. High dietary diversity and the consumption of high trophic value prey were observed in birds' pre‐breeding and breeding diet and likely led to early breeding and high reproductive success. Our results highlight that resource abundance and the availability of a variety of prey taxa are critical factors in enabling this inshore seabird to adjust to changes in environmental conditions and fluctuations in prey. An understanding of seabird diet is integral to their conservation and management. Monitoring seabird trophic niche dimensions and reproductive parameters can elucidate causes for population declines and can provide information about particular prey species and foraging locations that require protection.