Birds That Build Nests With Domes May Be Doomed

Birds That Build Nests With Domes May Be Doomed

Many of the bird nests you’ll spot this spring will have the familiar open and cupped shape, perfect for securing eggs and eventually hatchlings. About 30 percent of bird species are the starchitects of the avian kingdom, constructing elaborate domed nests with roofs. While ecologists have long thought that domed nests provided greater safety from predators and weather, a new study suggests songbirds who opt for simpler nests may be better off in the long run.

Almost all songbirds can be traced to Australasia around 45 million years ago, when Australia was attached to Antarctica and covered by lush forests instead of parched deserts. Statistical analyses of songbirds’ traits and evolution found domed nests were the “ancestral architecture” of songbirds’ homes. But domed nests were then abandoned in favor of simpler cup designs when songbirds began spreading around the rest of the world some 40 million years ago.

Evolutionary biologists, like Iliana Medina of the University of Melbourne, wondered why domed nests were abandoned by so many modern birds, and why only a third of birds build them today. To answer that, she and her colleagues examined the ecological success of dome builders compared with cup builders, then tied those data to their evolutionary history.

For over 3,100 songbird species, Dr. Medina and colleagues gathered as much data as they could find: how big the birds’ bodies and ranges are, their latitude and elevation, whether they live in cities and, of course, what kind of nests they build. All this information was necessary because many factors influence how successful a species is, and Dr. Medina wanted to home in on nest type as precisely as possible.

Her analyses, published last month in the journal Ecology Letters, revealed surprising patterns. Songbirds that build domed nests tend to have smaller ranges, with stricter climatic needs. If domed nests offer better protection, some ecologists had thought, that could allow birds’ ranges to expand and withstand broader conditions. Dr. Medina’s results contradict that thinking.

Based on the findings, Dr. Medina proposes that dome builders might be less adaptable than cup builders. Though domed nests offer better protection from the elements, they also tend to be larger — easier for a predator to spot. Bigger nests also take more time to build and require more materials, potentially limiting both when and where they could be built and making birds less likely to leave an imperiled habitat, like a sunk-cost fallacy with feathers.

“Maybe it’s actually better to have a disposable, cheap nest that you can build multiple times a season,” said Jordan Price, an evolutionary biologist at St. Mary’s College of Maryland who was not involved in the study. “You’re exposed to the elements, but you can escape predators really quickly.”

The research also showed that dome builders are less likely to live in cities, perhaps because of a lack of suitable nesting sites, a dearth of building materials or even because cities tend to be warmer. Dome builders also take longer to build nests, an intuitive finding that had not been supported by a global analysis until now.

Dr. Medina then peered back in time, modeling the natural history of nest-building traits and new species over songbirds’ roughly 45-million-year history. She found dome builders had slightly higher extinction rates than cup builders, a result contrary to notions that domed nests were safest.

“The cost-benefit analysis of building either an open-cup nest or a dome nest changed at some point,” Dr. Price said. “Some species kept their old ways, and some innovated something new, which allowed them to really flourish.” What prompted the change of cost, however, remains unknown; new parasites or predators could have arrived, or climates could have changed.

Today, dome builders face new challenges posed by humans, including variable climates, habitat loss and built environments. Birds, like many other fauna, are experiencing accelerating rates of extinction.

“There aren’t any real management actions we can do about the nest of a species,” said James Mouton, a postdoctoral fellow at the Smithsonian Migratory Bird Center who was not involved in the study. “It’s not something we can coach them in.” But conservation efforts could help restore and protect important dome-nester habitats, bolstering potentially vulnerable populations.

“There are some pretty ancient lineages, some birds that branched off the songbird tree really early,” Dr. Price said. “We need to look out for these species.”

He added, “Some of these dome-nesting species, it would be terrible to lose.”

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