A new study has found that birds build hanging nests, particularly those with extended entrance tunnels, to help protect offspring against invaders like snakes and parasitic cuckoos.
Researchers at Durham University, the British Trust for Ornithology and Princeton University examined the relationship between nest design and the length of time offspring spend in the nest before fledging across several species of weaver and icterid, two bird families renowned for their complex woven nests.
They found that species building the most elaborate nests, particularly those with long entrance tunnels, produce offspring with longer developmental periods. Nests with longer entrance tunnels are more effective at hindering access by nest invaders than shorter tunnels and thereby limiting the exposure of developing offspring to nest invaders.
Southern Masked Weaver is one of numerous African species that builds hanging nests (Peter Warne).
Researchers suggest that the complex structural features in these nests do indeed play a role in protecting offspring from predators and brood parasites.
They described the consistency of these findings as 'striking' given that highly elaborate nests have evolved independently in the weavers and icterids.
Dr Sally Street of Durham University, lead author of the study, said: "Ornithologists have long been fascinated by the beautifully woven nests of weavers and icterids – these nests often dangle precariously from slim branches and some have extended entrance tunnels up to a metre long.
"It has been widely assumed that these nests prevent attacks by tree-climbing snakes but this idea is largely based on anecdotes until now. We are excited to show that these ideas appear to be correct – species building the most elaborate nests, particularly those with long entrance tunnels, have more slowly developing offspring which is exactly what we should expect if the nests protect chicks from predators and other nest invaders such as brood parasitic cuckoos"
The researchers also revealed that by building protective structures such as the elaborate nest, birds and other species can deploy greater control over their exposure to environmental hazards.
The scientists obtained data on nest design, life history traits, body mass and latitude in weaver and icterid species from multiple secondary sources.
Their findings reveal how animal architects such as nest-building birds and burrowing mammals can create protective environments that change how their offspring develop.
The researchers say this may even help to understand the role of shelter-building in human evolution.
Street, S E, Jacques, R, & De Silva, T N. 2022. Convergent evolution of elaborate nests as structural defences in birds. Proceedings of the Royal Society B. DOI: https://doi.org/10.1098/rspb.2022.1734