Nest attachment, rather than nest type, correlates with passerine bird brain size

Variation in relative interspecific brain size has been correlated with cognitive capacities in different animal groups. Bird nest construction is one of the most remarkable animal abilities, and has reached the highest diversification in the Passeriformes. Yet, its relationship with brain size is not fully understood. Here, we used a dataset of 455 species to address potential correlations between nest types (open and enclosed) and five categories of nest attachment mode, as well as a set of covariables, with relative brain mass (Rbmass) of passerine birds. Bayesian regression modelling with phylogenetic control revealed that nest attachment mode, rather than nest type, was associated with Rbmass variation, despite the strong effects of habitat, migration and phylogeny. A phylogenetic confirmatory path analysis suggested that Rbmass and nest attachment can interact via a direct evolutionary link and also through an indirect link mediated through habitat (vegetation density). Phylogenetic ridge regression indicated that Top suspended nests were associated with species close to a maximum relative brain size, and that Rim suspended nests were associated with brain size radiations and probably with the exploration of new ecological niches. Our study provides evidence that the construction of nests with different attachment modes requires different levels of cognitive abilities, and we provide insights into the relationships between passerine brain size and nest attachment diversification.