A wide range of abiotic and biotic variables leaves most variation in bird nest architecture unexplained.

Nests are the locations or containers for offspring, and mediate interactions between offspring and the environment. However, understanding how environmental factors shape the evolution of nest architecture is complicated. In particular, the relative contributions of biotic (e.g. protection from predation) and abiotic (e.g. microclimate maintenance) factors to the evolution of nest architecture have not been clearly quantified, and multiple nest traits, such as their location or shape, are rarely considered together. Here, we use a dataset of 3685 bird species for which data exist across multiple nest traits to characterize a multivariate 'morphospace' of nest architecture and use phylogenetic comparative methods to explore whether abiotic or biotic factors better explain this variation. We detect that abiotic environmental factors (climate) explain more variation than biotic factors. However, substantial variation in nest architecture remains unexplained after accounting for the variables used here, suggesting that commonly used nest traits may not capture covariation between nest architecture and the environment as expected. Nonetheless, our study demonstrates how nest evolution is affected by the environment on a global scale. This provides a foundation to explore a more diverse array of nest traits and environmental variables, to better understand nest evolution in the world's birds.