How to protect your chicks from predators? Build a dome over them! There is tremendous diversity among the nests of birds, in nest location, structure, materials, and more, but we know very little about the forces that shaped the evolution of this incredible variety.

In a new paper published this week in The Auk: Ornithological Advances, Zachary Hall, Sally Street, Sam Auty, and Susan Healy of the University of St. Andrews in Scotland test the hypothesis that domed-shaped nests arose as a result of some species transitioning to nesting on the ground, where the risk from predators is greater.

Hall was completing his Ph.D. work on the neurobiology of nest-building behavior when he noticed that very little work had been done on trying to understand why different bird species build such drastically different nest structures.

"I thought this was strange," he explains, "because the shape of a nest seems to be the most striking and diverse feature across bird species."

The hypothesis that dome-shaped nests resulted from the increased predation risk when competition for nest sites led some birds to begin nesting on the ground was first proposed almost twenty years ago, but techniques at the time did not provide a way to test it.

Applying statistical techniques he had previously used in his neurobiology study, Hall and his colleagues collected previously published descriptions of the nests of 155 species of babbler and mapped nest height and structure to the birds' family tree.

Their analysis confirmed that babblers' ancestors likely built above-ground, cup-shaped nests, and that the addition of a dome to cover the nest corresponded with switching to nesting at ground level.

"This new study by Hall, Street, Auty, and Healy looks at the evolution of two key aspects of animals as architects: how they shape their homes and where they put them. It shows very nicely how we can take advantage of recent progress in avian phylogenetics to test ideas about the evolutionary history behind the modern-day co-occurrence of particular pairs of traits," according to Don Dearborn, an expert in the evolution of reproductive strategies in birds.

"I am very happy how well nest structure integrated into our analyses, but this study is only the tip of the iceberg, and we hope future work can use a similar approach to identify other factors that may have influenced the evolution of nest structure," adds Hall.

The coevolution of building nests on the ground and domed nests in Timaliidae