Egg mass drives the evolution of bird nest architecture.

Although most birds build nests, not all of them build nest walls, and the evolutionary advantage conferring this architectural feature remains inconclusive. By integrating macro-evolutionary patterns with individual fitness consequences, we investigated the function of nest walls in protecting eggs and the evolutionary driver behind this construction. We examined two novel hypotheses relating to egg rolling-off risk: (i) heavy-bird hypothesis: heavier birds generate greater nest vibrations during attendance, increasing egg rolling-off risk; and (ii) light-egg hypothesis: lighter eggs are more susceptible to displacement from nest vibrations due to lower friction against the substrate. Phylogenetic comparative analyses across 4030 species revealed that birds with lower body and egg mass were more likely to construct nest walls, suggesting higher rolling-off risk for lighter eggs. Experiments combining behavioural observations and controlled trials with three-dimensional-printed eggs confirmed that lighter eggs were more prone to falling from nests without walls. The evidence across phylogeny and individual-level experiments consistently supports the light-egg hypothesis, suggesting the adaptive function of nest walls in preventing eggs from falling during external disturbance, particularly for lighter eggs. This study demonstrates how integrating macro- and micro-evolutionary approaches can reveal the functional mechanisms underlying correlated evolution between phenotypic traits.