Data on Tree Swallow (Tachycineta bicolor) body mass, wing, and headbill length

Body-size reductions are a pervasive response to climate change, and body size is a central trait linking together multiple axes of ecology, physiology and life history. Using a combination of three decades of data and controlled experiments, we show that male and female tree swallows (Tachycineta bicolor) have become smaller structurally, despite chicks growing larger under warmer nest temperatures and larger chicks being more likely to return as adults. We find that adult structural size trends are associated with warmer overwintering conditions, rather than the nestling period. Further, adult male body mass trends depend on climate conditions during spring migration; male breeding mass decreased by 4%, whereas female mass was unchanged. This may be explained by the demands of reproduction, as lighter females produce fewer offspring. This work highlights the complex interactions that shape relationships between traits and fitness, which will be critical for predicting evolutionary responses in future environments.