Do older parents do better? Relationships between parental age, chick body condition and migratory behaviour in a colonial-breeding waterbird

Animals' performance of basic functional behaviours, such as foraging and movement, may improve with age as a result of past experiences. In migratory birds, for example, due to earlier or more efficient migration, older and likely more experienced individuals tend to arrive at breeding sites earlier and enjoy better breeding conditions than younger conspecifics, resulting in a higher reproductive success. Yet, despite the advantages of early arrival for breeding adults, the long-term effects of fledging early and/or with a higher body condition on chicks' future fitness prospects remain largely unexplored. In differential migration systems, low-quality or socially subordinate individuals may be constrained to sub-optimal migratory behaviours associated with lower demographic rates. Therefore, producing high-quality chicks may enhance the survival of offspring. In this study, we analysed data from the long-term ringing programme on the breeding population of Eurasian spoonbills in the Camargue (southern France) to investigate how breeder age may influence the timing of breeding and, in turn, how this may affect chick body condition and their subsequent migratory behaviour. Using breeding resightings of birds individually marked as a chick since 2008, combined with chick biometric measurements and subsequent winter resightings of offspring, we show that older spoonbills tend to breed earlier in the season than younger individuals, and that early breeders, regardless of age, are more likely to produce chicks with higher body condition than late breeders. Finally, migratory behaviour of juveniles appears to be influenced by the timing of breeding, with later-born juveniles tending to undertake less demanding migrations (without crossing major ecological barriers) than juveniles born earlier in the breeding season. Our study therefore highlights the relevance of long-term studies to better understand the complex breeding phenology of migratory species, which can lead to changes in population-level patterns and processes.