North American songbirds have been shrinking steadily in size over the past 40 years, according to scientists who measured tens of thousands of the feathered creatures from dozens of different species and attributed the changes to rising temperatures.As the birds’ bodies got smaller, their wings gradually got longer, the scientists said in a paper published Wednesday in the journal Ecology Letters. The longer wings, the researchers said, may help offset the loss of body mass so the birds can fly efficiently on their long migrations.
The changes were too small to be apparent to the naked eye, the scientists said, amounting to a gram or so in weight per bird and a few millimeters change in individual wing length. “But we see them consistently across so many species, across male and female, across ages and all these things that can introduce variability,” said Benjamin Winger, a University of Michigan evolutionary biologist who was the senior scientist on the project. “Almost all signs point to the body size getting smaller while the wing is getting longer.” The findings were based on measurements of more than 70,000 birds collected since 1978 at the Field Museum of Natural History in Chicago. The birds belonged to 52 different species.
The birds used in the study died from collisions with high-rise buildings in Chicago.
Photo:
University of Michigan
“Despite the diversity of so many species, they are all changing in the same way,” said Brian Weeks, an evolutionary ecologist at the University of Michigan who was the lead author of the study. “We can say with a fair amount of confidence that the changes are associated with increasing temperatures.” Warm-blooded animals are generally larger in cold climates and smaller in warm climates because more compact creatures usually release heat more quickly, according to biologists and ecologists. Given the well-established link, many scientists had predicted in recent years that global warming would affect the size of many animals. Yet until recently, there wasn’t much evidence of the effect at work during modern warming trends. The new findings are the latest in a series of technical reports this year that link changes in body size among birds to warmer temperatures around the world. Last month, researchers in Australia who studied physical changes in 82 songbird species, including honeyeaters, fairy-wrens and thornbills, reported in the Royal Society B journal that birds there have grown smaller due to warming over the last half-century, as the annual mean temperature increased regionally by about 0.012 degrees Celsius. They based their conclusions on an analysis of 12,000 museum specimens.
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In March, researchers at the University of Cape Town in South Africa who tracked the weight of a long-tailed songbird common across Africa called the mountain wagtail found the species gradually became lighter between 1976 and 1999, as regional temperatures increased by 0.18 degrees Celsius. They published their findings in the journal Oecologia. Migrating birds in the modern world face many hazards affecting their growth and survival, from vanishing nesting grounds, dwindling food sources and pesticide use, to domestic cats, which kill up to 3 billion birds annually, according to the Migratory Bird Center at the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service. Collisions with high-rise buildings kill another 600 million or so migrating birds every year, according to the Cornell Lab of Ornithology at Cornell University. The thousands of sparrows, warblers, thrushes and other birds analyzed in the new study were all killed when they collided with high-rise buildings in Chicago between 1978 and 2016, the scientists said. For the latest study, David Willard, a Field Museum ornithologist, measured the length of the leg, the wing, the beak and the weight of each bird carcass kept at the museum. The scientists, analyzing the measurements, found that body size declined significantly over the 40 years, as temperatures at the birds’ summer breeding grounds north of Chicago increased by about one degree Celsius. The length of the tarsus leg bone, a key indicator of overall body size, declined 2.4% across all 52 species during those decades. The birds’ sizes declined incrementally, generation by generation, as temperatures rose, the researchers found. At the same time, wing length increased 1.3% in 40 of the species, the scientists said. Write to Robert Lee Hotz at sciencejournal@wsj.com
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