Carry-over effects of brood size on morphology, reproduction, and lifespan in barn swallows

Early life ecological conditions have well-documented short-term effects on offspring phenotype and survival, but the extent to which these effect carry-over into adulthood is much less known. Yet, unveiling such carry-over effects is essential to understand the evolution of parental life-history strategies. In altricial birds, the number of brood mates can affect competition regime and other nest ecological conditions, whose effects may be at least partly expressed in adulthood. We either increased or decreased the size of barn swallow (Hirundo rustica) broods and analyzed the consequences of brood size manipulation on morphological feather traits, breeding performance, and lifespan of the offspring when adult. Upon recruitment (age 1 year), individuals from enlarged broods had shorter wings, later reproduction, and lower breeding output than those from reduced broods. The negative effect of brood enlargement on wing length persisted at age 2 years. Recruits from enlarged broods had longer lifespan but the proportion of nestlings that were recruited tended to be smaller for enlarged compared to reduced broods. Hence, large brood size had negative phenotypic effects in adulthood. Our results also suggest that stronger viability selection on offspring from enlarged broods results in differential survival of highly viable offspring that express longer life expectancy when adult and/or that smaller reproductive effort of 1-year-old offspring from enlarged broods boosts their life expectancy, potentially compensating for reduced annual fecundity. Number of brood mates can thus have carry-over effects on fitness components, including lifespan, which should be incorporated in the analysis of complex reproductive trade-offs. Significance statement: Ecological conditions in early life can affect survival and physical conditions but the extent to which these effects extend into adulthood is largely unknown. In this experiment, we altered the size of barn swallow broods and monitored the consequences of the change in competition regime in the brood of rearing in adulthood. When adult, individuals that had been reared in an enlarged brood had shorter wings and decreased breeding success. However, individuals from enlarged broods lived longer, possibly because only high-quality individuals from such broods survived through the first year of life. Thus, rearing conditions have important long-term effects, implying that parents have to optimize the number of offspring they decide to produce.