The causes and consequences of interspecific variation in sex-specific contributions to animal parental care are relatively well understood during pregnancy or incubation and during offspring provisioning, but comparative patterns of sex-biased investment during nest-, den-, or other shelter-building have been almost completely overlooked. This is surprising because birthing shelter properties have important fitness consequences for both parents and offspring. Here, we address this gap in our knowledge by testing predictions concerning sex-specific contributions to avian nest building in more than 500 species of Western Palearctic birds in relation to the time available to breed and sex-specific reproductive effort, whilst also examining correlates with nesting site and nest structure. Using multivariate phylogenetic comparative and path analysis approaches, we found that species in which females build nests alone have shorter breeding seasons and breed at higher latitudes. In addition, species in which females lay larger clutch sizes and incubate eggs alone are more likely to have nests built by females alone, implying that reproductive contributions are not traded-off between the sexes. Finally, sex-specific nest building contributions were related to nest site and structure, as species in which females built nests alone were more likely to have open cup nests relative to domed nests of species in which both parents build. Our study provides important new insights, and generates several new questions for experimental research into the adaptive dynamics of sex-specific contributions prior or at the onset of breeding.

We quantified nest building contributions and several life history and ecological traits for 521 bird species. Nest building contributions were classified by coding the species’ descriptions in the Birds of the Western Palearctic book series and were classified as being ‘neither’ if no birds collected materials, ‘male’ if males build the nest alone, ‘female’ if females build the nest alone and ‘both’ if males and females build the nest together. Nest sites were classified as being grass/reed, ground, ground hole/cavity, ledge, tree/bush, tree hole/cavity, wall or water. Nest complexity/structure was classified following the approach of Hall et al. (2013), although we added an extra ‘dome’ category to separate the domed nests from the cup nests of species. We defined clutch sizes as the means of the upper and lower average values of the published clutch sizes. The sex that incubates was defined as being ‘male’, ‘female’ or ‘both’. The duration of breeding was classified as the number of months between the first and last month of breeding. We used the mean average of the maximum and minimum breeding ranges for measuring species' mean breeding latitude values.