No apparent trade-off between the quality of nest-grown feathers and time spent in the nest in an aerial insectivore, the Tree Swallow

Life history theory provides a framework for understanding how trade-offs generate negative trait associations. Among nestling birds, time spent in the nest, risk of predation, and lifespan covary, but some associations are only found within species while others are only observed between species. A recent comparative study suggests that allocation trade-offs may be alleviated by disinvestment in ephemeral traits, such as nest-grown feathers, that are quickly replaced. However, direct resource allocation trade-offs cannot be inferred from interspecific trait associations without complementary intraspecific studies. Here, we asked whether there is evidence for a within-species allocation trade-off between feather quality and time spent in the nest in Tree Swallows (Tachycineta bicolor). Consistent with the idea that ephemeral traits are deprioritized, nest-grown feathers had lower barb density than adult feathers. However, despite substantial variation in fledging age among nestlings, there was no evidence for a negative association between time in the nest and feather quality. Furthermore, accounting for differences in resource availability by considering provisioning rate and a nest predation treatment did not reveal a trade-off that was masked by variation in resources. Our results are most consistent with the idea that the interspecific association between development and feather quality arises from adaptive specialization, rather than from a direct allocation trade-off. Lay Summary center dot For many species of birds, nestlings hatch undeveloped and enter a period during which they must grow rapidly in the nest while also being exposed to a high risk of predation. center dot This intense period of growth can create strong trade-offs where a limited pool of resources must be invested into a variety of traits simultaneously. center dot One way that nestlings might balance these trade-offs is by withholding resources from traits that will have less influence on successful survival into adulthood. center dot We studied wild Tree Swallows to ask whether there is evidence for this direct investment trade-off by measuring fledging age, feather quality, and morphology while manipulating resource availability. center dot We did not find evidence for a direct trade-off within Tree Swallows, and we discuss how this result informs the interpretation of previous work comparing feather quality and development between species.

Automated summary

Life history theory provides a framework for understanding how trade-offs generate negative trait associations. Among nestling birds, time spent in the nest, risk of predation, and lifespan covary, showing the importance of understanding the allocation trade-offs between different traits in avian development.