Ecological and life history factors shape the complexity of bird nests

Abstract Nest complexity varies greatly, indicating that it represents a significant adaptive trait in avian evolution. However, the mechanisms underlying the evolution of nest complexity are not fully understood. Hypotheses involving environmental factors, cognitive ability, parental investment, movement ability, and brood parasitism have been proposed to explain the mechanisms that drive the evolution of nest complexity. In this study, we compiled data on the characteristics of nest structures from 5,028 bird species spanning the avian phylogeny, as well as environmental factors (including nest site, annual mean temperature, annual mean precipitation, latitude, habitat density), cognitive ability (relative brain size), parental investment (clutch size, incubation duration, and developmental mode), movement ability (migratory behavior and dispersal ability, quantified using the hand-wing index, HWI), and brood parasitism (host status). We identified significant driving factors of nest complexity using Bayesian phylogenetic mixed models and we addressed direct and indirect causal effects in the relationships among these significant driving factors using phylogenetic path analysis. Our results indicated that nest complexity was significantly correlated with nest site, annual mean precipitation, latitude, relative brain size, incubation duration, developmental mode, and HWI. Additionally, nest site, relative brain size and HWI had a direct impact on nest complexity, suggesting that multiple mechanisms drive global bird nest complexity. Furthermore, we provide the first evidence that birds with lower dispersal ability tend to construct more complex nests on a global scale. Our findings will enhance the understanding of the evolution of nest complexity.