Unpacking the lifelong secrets of little penguins: individual quality, energy allocation, and stochasticity in defining fitness.

While the heterogeneity among individuals of a population is more and more documented, questions on the paths through which it arises, particularly whether it is linked to fixed heterogeneity or chance alone, are still widely debated. Here, we tested how individual quality, energy allocation trade-offs, and environmental stochasticity define individual fitness. To do so, we simultaneously investigated the contribution of 18 life-history traits to the fitness of breeding little penguins (Eudyptula minor), using a structural equation model. Fitness was highly variable amongst the 162 birds monitored over their entire lifespan. It increased with the individual penguin's ability to increase (a) the number of breeding events (i.e., living longer, breeding younger, breeding more often, and producing more second clutches) and (b) the breeding success per event through increased foraging performances (i.e., mass gained at sea). While all three processes (stochasticity, individual quality, and allocation trade-offs) affected fitness, interindividual variability in fitness was mainly driven by individual quality, birds consistently breeding earlier in the season and displaying higher foraging efficiency exhibiting higher fitness. Why some birds consistently can perform better at sea and breed earlier remains a question to investigate to understand how selection applies to these traits.