Convergent evolution of elaborate nests as structural defences in birds

Abstract

The hanging-basket nests of some weaverbird and icterid species are among the most complex structures built by any animal, but why they have evolved remains to be explained. The precarious attachments and extended entrance tunnels characteristic of these nests are widely speculated to act as structural defences against invasion by nest predators, particularly tree-climbing snakes, but this hypothesis has yet to be systematically tested. We use phylogenetic comparative methods to investigate the role of nest structure in developmental period length, a proxy for offspring mortality, in the weaverbirds (Ploceidae) and icterids (Icteridae), two bird families in which hanging-basket nests have independently evolved. We find that more elaborate nest designs, particularly those with entrance tunnels, are associated with longer developmental periods in both families. This finding is robust to potentially confounding effects of body mass, phylogenetic relationships and nest location. We provide the first comparative evidence for the role of elaborate nest designs in reducing offspring mortality from nest invasion. More broadly, our findings highlight the importance of offspring mortality risk in shaping diversity in birds nest designs and suggest that complex structures built by animals can buffer against environmental hazards and facilitate the evolution of slower life histories.