In 1978, David Willard, an ornithologist at the Field Museum in Chicago, began collecting birds that had died after colliding with large windows—a fate that befalls up to one billion birds each year. Over the decades, Willard worked with volunteers from the Chicago Bird Collision Monitors during the spring and fall migration seasons to assemble a collection of more than 100,000 specimens. He kept meticulous measurements of the dead creatures, with the goal of tracking seasonal trends.
In a new statistical analysis of this vast trove of data, Willard and his fellow researchers reveal an important trend that emerged among the specimens over time. Writing in the journal Ecology Letters, the team explains that the bodies of the birds in the Field Museum’s collections have gotten smaller, while their wingspans have increased in size—a morphological phenomenon that the scientists attribute to climate change.
The new study looked at 70,716 individual specimens, representing 52 bird species, that had been collected between 1978 and 2016. “Most breed in boreal or temperate forest or edge habitats, but some species are grassland or marsh specialists, and their winter ranges, habitats, migratory distances, life histories and ecologies are diverse,” the study authors write in the study.
Researchers found that the birds’ lower leg bone—a common indicator of body size, according to the BBC’s Kelsey Vlamis—shrunk by 2.4 percent across all species. The birds’ masses also decreased, but their wingspans increased by 1.3 percent.
The researchers looked at several factors that might be driving these changes, among them temperature, precipitation and vegetation in the species’ breeding and wintering grounds. They found an inverse relationship between body size and summer temperature. Or in other words, “[i]n years when temperatures were a bit warmer, the bodies got smaller,” Benjamin Winger, study co-author and evolutionary biologist at the University of Michigan, tells Ben Guarino of the Washington Post.
“In years when temperatures were a bit cooler, we saw an increase in body size, even though the long-term trend was to decline,” adds Winger. “So that leads us to believe that temperature is pretty important here.”
This isn’t an entirely new idea. Within a given species, individuals that live in warmer climates tend to be smaller than their counterparts in colder areas, because small bodies retain less heat—a principle known as Bergmann’s Rule. The scientists suspect that climbing temperatures may be driving birds to become smaller, which in turn helps them stay cool.
But while the researchers suspected that rising temperatures would be linked to a reduction in body size, the consistency of the trend among winged critters of diverse natural history, habitats and geographic ranges was “shocking,” says Brian Weeks, lead study author and evolutionary biologist at the University of Michigan. “I was incredibly surprised that all of these species are responding in such similar ways.”
The increasing wingspans were another intriguing phenomenon. Reduced body size means that birds have less energy to complete long and taxing migrations, Weeks tells the BBC's Vlamis. Perhaps, the study authors hypothesize, longer wings represent an adaptation that helps birds compensate for their smaller bodies.
It is not yet clear whether these changes are harmful to birds, but the study suggests that animal size and shape are an important consideration when predicting how species will react to a continually warming planet. The paper also highlights the value of Willard’s tremendous dataset, which he began assembling without knowing how the collection would one day be used.
"When we began collecting the data analyzed in this study, we were addressing a few simple questions about year-to-year and season-to-season variation in birds,” says Willard, now a collections manager emeritus at the Field Museum. “The phrase 'climate change' as a modern phenomenon was barely on the horizon. The results of this study highlight how essential long-term data sets are for identifying and analyzing trends caused by changes in our environment.”