Individual quality and age affect responses to an energetic constraint in a cavity-nesting bird

Individual variation in life-history trade-offs can be caused by differences in quality and age. We tested for individual variation in parental investment in incubating tree swallows (Tacyhcineta bicolor) subjected to a feather-clipping manipulation. Individual quality influenced how females were affected by feather clipping; lower quality clipped females showed a greater reduction in incubation and a greater loss of body condition than higher quality clipped females compared with controls. Most importantly, responses during incubation influenced nestling traits; lower quality clipped females, particularly those losing the most body mass, raised nestlings in the poorest condition. There was no difference in incubation patterns of control females, but older clipped females suffered self-maintenance costs and raised offspring in better condition. In contrast, younger clipped females passed costs on to offspring through lower egg temperatures and reduced nestling condition while maintaining their own condition. Overall, we found a strong individual quality effect: at the population level, there was a trade-off between investing in incubation and maintaining parental condition, but among individuals, there was a positive correlation between change in parental condition and offspring quality. Individual differences in parental strategy can be important causes of life-history variation, especially through subtle, but important, aspects of reproduction such as maintaining egg temperature during incubation.