The fecundity costs of building complex nests in birds

Animal nests provide a beneficial environment for offspring development and as such contribute to fitness. Gathering and transporting materials to construct nests is energetically costly, but the life history trade-offs associated with the complexity of nests built are largely unknown. Who contributes to building the nest could also mediate these trade-offs, as building a nest as a couple is expected to be less costly per individual than building alone. Using a comparative analysis on 227 songbird species globally, we found a fecundity cost associated with the type of nest a species builds. Species that build complex dome nests produce fewer broods per year than species building more simple cups or platforms. On the other hand, dome nesting species have larger clutch sizes than open nesting species, but only when the nest is built by a couple and not when females build nests alone. This suggests that building dome nests represents a trade-off with investment in young, especially when females are solely responsible for nest building. More broadly, our results could explain macroevolutionary patterns, such as the recent finding that females more often build open cup rather than dome nests.