Birds that build nests with domes can be doomed

Many of the bird nests you will notice this spring will have the familiar open and cup shape, ideal for securing eggs and eventually hatching. About 30 percent of the bird species are the architects of the bird kingdom, who build complex domed nests with roofs. While environmentalists have long believed that domed nests provide greater safety from predators and weather, a new study suggests that songbirds that choose simpler nests may be better off in the long run.

Almost all songbirds can be traced back to Australasia about 45 million years ago, when Australia was attached to Antarctica and covered with lush forests instead of dried-up deserts. Statistical analyzes of songbird traits and evolution have shown that domed nests are the “ancestral architecture” of songbird homes. But domed nests were abandoned in favor of simpler cup designs when songbirds began to spread around the world about 40 million years ago.

Evolutionary biologists, such as Iliana Medina of the University of Melbourne, have wondered why dome nests have been abandoned by so many modern birds and why only a third of birds build them today. To answer this, she and her colleagues studied the environmental success of dome builders over cup builders, and then linked the data to their evolutionary history.

For more than 3,100 species of songbirds, Dr. Medina and his colleagues collected as much data as they could find: how large the bodies and ranges of birds are, their latitude and altitude, whether they live in cities and, of course, what nests have built All this information was needed as many factors affect how successful the species is and Dr. Medina wanted to establish the type of nest as accurately as possible.

Her analysis, published last month in the journal Ecology Letters, revealed surprising patterns. Songbirds that build domed nests usually have smaller ranges, with more stringent climatic needs. If domed nests offer better protection, some environmentalists say, it could allow the birds to expand and survive in wider conditions. Dr. Medina’s results contradict this thinking.

Based on the findings, Dr. Medina suggests that dome builders may be less adaptable than those who build cups. Although domed nests offer better protection from the elements, they are also larger – easier for a predator to notice. Larger nests also take longer to build and require more materials, which potentially limits when and where they can be built and makes birds less likely to leave an endangered habitat, misleading by the sinking costs of feathers.

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

The study also found that dome builders are less likely to live in cities, perhaps because of a lack of suitable nesting sites, a shortage of building materials, or even because cities tend to be warmer. Dome builders are also taking more time to build nests, an intuitive discovery that has so far not been supported by global analysis.

Dr. Medina then looked back in time, modeling the natural history of nesting features and new species on the approximately 45 million history of songbirds. She found that dome builders have a slightly higher rate of extinction than those who build cups, a result that contradicts the notion that dome nests are the safest.

“The cost-benefit analysis of building a nest with an open cup or dome nest has changed at some point,” said Dr. Price. “Some species have kept their old ways, and some are innovating something new that has allowed them to really thrive. What caused the price change, however, remains unknown; new parasites or predators may have arrived or the climate may have changed.

Today, dome builders face new challenges posed by humans, including changing climates, habitat loss and the built environment. Birds, like many other fauna, are experiencing an accelerated rate of extinction.

“There is no real management action we can take on a nest of a species,” said James Mouton, a doctoral student at the Smithsonian’s Migratory Bird Center who was not involved in the study. “It’s not something we can teach them.” But conservation efforts can help restore and protect important nesting habitats by increasing potentially vulnerable populations.

“There are some pretty ancient lines, some birds that branched out from the songbird tree really early,” Dr. Price said. “We have to be careful about these species.”

He added: “Some of these nesting dome species would be horrible to lose.”

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