Which came first, the cup or the dome? Unlike the old chicken-versus-egg conundrum, this question appears to have an answer. A new study suggests that the familiar open-cup style—built by nearly three fourths of today's passerines, or perching birds—is a modification of roofed spherical structures that just a handful of species now make.

Most biologists had theorized that nest shape evolved the other way around, from bowl to dome. Researchers recently tested the hypothesis by overlaying nest-structure data on three different phylogenetic trees, thought to represent the evolutionary relations among 281 Australian passerine species. The team noticed that species with particularly ancient lineages, such as lyrebirds, scrub-birds and New Zealand wrens, still build roofed structures—suggesting that ancestral passerine nests were domed. A statistical analysis of the likelihood that particular nest shapes occurred in ancestors confirmed the hunch: the dome came first.

Cup Nest
Cup-shaped nest of a chipping sparrow (Spizella passerina). Credit: Sharon Beals

The researchers also found that making cup-shaped nests evolved multiple times and in different lineages; 187 of the studied species build them today. The results were detailed in February in the Proceedings of the Royal Society B.

Cups may offer some advantages, such as being easier to build or to abandon if predators approach. “I think most people had assumed that roofed nests evolved from cups, in part because roofed nests are so unusual today,” says co-author J. Jordan Price, a professor of biology at St. Mary's College of Maryland. “This nicely illustrates how the current prevalence of a trait, such as cup nests, does not necessarily indicate the order of events during its evolutionary history.”

The findings could inform how scientists study nest evolution, says Gavin Leighton, an evolutionary biologist at the Cornell Lab of Ornithology, who was not involved in the study. “I think there will be increased interest in determining the ecological scenarios that select for different nest types,” he says. Seems you can't put all your nests in one basket.