An impressive 600 million birds die each year in the United States after colliding with tall buildings. And Chicago, with its skyscrapers and location on a major migration path, is perhaps the biggest killer.
But for Dave Willard, emeritus manager of the city's Field Museum collections, the dead birds were an unexpected unexpected scientific result. Every morning, in spring and fall, when the birds make their epic journey between Canada and Latin America, he goes out to fetch the dead animals from the street.
"I just stopped for the season. The number of birds we receive every day is highly variable, depending on whether it is a large migration day. The maximum is 300 in one day," he said.
Together with a group of volunteers, since 1978 he has collected over 100,000 dead birds, carefully measuring them with caliber and scale and cataloging the results manually in a book.
Now, a comprehensive study of Willard's unique and remarkably detailed data has shown that US migratory offerings have been declining over the past four decades and are far larger. The changes seem to be a response to the hot weather.
"We had good reason to expect that rising temperatures would lead to reductions in body size based on previous studies," said Brian Weeks, assistant professor at the University of Michigan School of Environment and Sustainability and lead author of the study published Wednesday. -market. in the journal Ecology Letters.
"What was shocking was the consistency. I was incredibly surprised that all these species are responding in a similar way," he added in a press release.
Warmer temperatures and smaller bodies
For the analysis, biologists used 70,716 dead birds representing 52 species – including thrushes, sparrows and warblers – which Willard recorded between 1978 and 2016. Of these species, 49 had statistically significant declines in body size. In particular, the length of the tarsus or leg bone decreased by 2.4%.
Meanwhile, wing length showed an average increase of 1.3%, with species showing the fastest declines in tarsal length, also showing the fastest gains in wing length.
The authors suggested that reduced body size is a response to climate warming, with temperatures at summer farms in northern Chicago increasing approximately 1 degree Celsius throughout the study.
Within animal species, individuals tend to be smaller in warmer parts of their range – a pattern known as the Bergmann Rule – the researchers said. Larger body sizes help animals in cold places get warm, with smaller bodies holding less heat.
The wingspan of birds may have increased to compensate for smaller bodies that produce less energy for the incredibly long distances that birds travel during their migrations.
Although changes in the body shape of birds are subtle and undetectable by the human eye, biologists said the study is the largest of its kind and shows the most consistent large-scale responses for a diverse group of birds.
"Periods of rapid warm-up are closely followed by periods of decline in body size and vice versa," said Weeks.
"Being able to show this kind of detail in a morphological study is unique to our work, as far as I know, and is entirely due to the quality of the data set that David Willard generated."
The possibility of reducing body size in response to current global warming has been suggested for decades, although the evidence supporting the idea remains mixed.
Next, the researchers plan to use the database to analyze the mechanism behind changes in body size and shape and whether they are the result of a process called developmental plasticity – an animal's ability to modify its development in response to a change. Environment change.
Willard began collecting dead birds after someone casually mentioned that they were knocking on windows, including McCormick Place, North America's largest convention center, just a mile away from the Field Museum. Willard said he went there thinking, "Why let them waste when they could be specimens in a museum?"
"When we started collecting the data analyzed in this study, we were addressing some simple questions about year-to-year and season-to-season variations in birds," said Willard. "The phrase & quot; climate change & quot; as a modern phenomenon was barely on the horizon."