Egg size and parental quality in thin-billed prions, Pachyptila belcheri: effects on offspring fitness

Seabirds laying a single egg per season can regulate breeding investment during prelaying through size and composition of the egg, and during postlaying by adjusting parental care. Both stages can influence offspring growth and survival. Although likely not independent, attempts to measure the relative contributions of egg size per se and parental quality to offspring fitness are rare. To unravel the effects of both variables, we performed a cross-fostering experiment in thin-billed prions, where pairs laying large or small eggs were randomly given either size of eggs to raise. Nestling growth was followed through to fledging. Hatchlings from large eggs were structurally larger and heavier. Egg size effects, for the first time shown to last throughout the nestling period, influenced chick growth parameters and fledging probability. Large-egg chicks reached a significantly higher maximum growth rate and peak mass than chicks hatched from small eggs. There was also a relationship between the size of the egg laid by the foster parents and growth of their fostered offspring. Chicks raised by high-quality parents were heavier and in better body condition shortly after hatching, when mortality is high, and were provisioned at a frequency that minimized mass fluctuations during growth. Significant year effects and year*egg size/parental quality interactions suggest that contributions of egg size and parental quality to offspring fitness are contingent on interannual environmental variability. We discussed maintenance of egg size polymorphism in this population, where egg size repeatability was found to be high.