Convergent evolution of elaborate nests as structural defences in birds

The pendent 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
relationship between nest structure and developmental period length, a
proxy for offspring mortality, in weaverbirds (Ploceidae) and icterids
(Icteridae), two bird families in which highly elaborate pendent nests
have independently evolved. We find that more elaborate nests,
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,
nest location and latitude. Our results are consistent with the hypothesis
that elaborate nest structures in birds can function as structural
defences, resulting in lower offspring mortality and slower development.
More generally, our findings suggest that constructing complex, protective
structures may buffer against environmental hazards, reducing extrinsic
mortality and contributing to the evolution of slower life histories in
diverse animal lineages, even humans.