Egg mass drives the evolution of bird nest architecture

Most birds build nests, but not all of them build nest walls. The functional advantage of such structure remains inconclusive. To understand the evolution of nest walls requires integrating functional mechanisms and behavioral adaptations, as both shape how organisms respond to environmental pressures over time. Here, by integrating the macroevolutionary patterns with the individual fitness consequences of building nests with walls, we aim to identify the evolutionary drivers of nest walls from the perspective of their function in protecting eggs. Specifically, we examined two novel hypotheses linking birds’ body or egg mass to nest structure mediated by the rolling-off risk of eggs. (i) Heavy-bird hypothesis: heavier birds can generate greater gravitational force, which may cause a higher degree of nest vibration, making eggs more likely to roll out of nests during nest attendance. (ii) Light-egg hypothesis: lighter eggs that have lower friction against the substrate and are less resistant to nest vibration caused by external forces. Through phylogenetic comparative analyses of 4,030 species, we found that birds—particularly those with lower absolute body and egg mass—were more likely to construct nest walls, suggesting that lighter eggs face a higher rolling-off risk due to their susceptibility to vibration. This finding aligns with our experimental results, which similarly showed that lighter eggs were more likely to fall off in nests without walls, supporting the light-egg hypothesis. The consistent evidence across both bird phylogeny and individual-level experiment suggests that nest walls serve as a functional adaptation, enhancing offspring survival by preventing egg loss. This study highlights the value of integrating macro- and microevolutionary approaches to better understand the functional mechanisms driving the evolutionary relationship between bird eggs and nest structures.