Birds are shrinking and climate change could be to blame

birds shrinking Photo by Karen Bean/Field Museum

It’s no small wonder: our warming climate seems to cause birds to shrink. Migratory researchers from the University of Michigan and the Field Museum in Chicago looked at more than 70,000 migratory bird specimens from 52 different species of North American birds and found a consistent decrease in body size.

Normally, we think of climate change endangering the survival of species in some locations, but these findings suggest that climate change may also lead to changes in the morphology—the form and structure—of wildlife. It also shows that long-term data collection is essential to understanding how the natural world is responding to our changing climate. 

birds shrinking
Photo by Roger Hart/University of Michigan Photography.

The study would not have been possible without the work of Dave Willard, an ornithologist at the Field Museum in Chicago. In 1978, Willard began collecting migrating birds that had fatally collided with buildings in Chicago for the museum’s ornithology collection. For every collected bird, he would record the specimen’s tarsus length (the tarsus is a bone found in the lower leg), bill length, relaxed wing length, and mass. 

When a team led by Brian Weeks, an assistant professor at the School for Environment and Sustainability at the University of Michigan, analyzed Willard’s 40-year stretch of data, it discovered a consistent decline in body size and an increase in wing length in 52 species of North American migratory birds.

“We can tie these reductions in body size to increased temperatures on the breeding grounds,” says Weeks. He proposes two possible mechanisms for the decrease in body size in relation to warming summer temperatures. One is that warmer temperatures cause nestlings to develop into smaller adults.

“The other possible mechanism is just natural selection. It could be that in warmer summers, smaller birds do better. So you see this shift in the populations towards smaller birds in warmer years.”

While the study examined 70,716 individuals, Willard has added a total of over 100,000 deceased birds from building collisions to the Field Museum’s collections. Weeks describes Willard’s specimen collection as totally selfless, a huge amount of work, and an incredible commitment to understanding the natural world.

birds shrinking
Photo by Ben Marks/Field Museum.

The four decades of work not only provided essential data about changes in bird body size, it also resulted in recommendations to reduce bird mortality from building strikes during the migration period. 

Willard now has a whole team of volunteers working with him to collect fallen birds. This volunteerism and public support is a must-have for long-term nature studies.

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As the natural world changes at an unprecedented scope and speed, public institutions like natural history museums are increasingly being called upon to answer questions about how species will respond to global change, says Weeks.

“Public support for basic science research is incredibly important,” he explains. “Even if it’s not a targeted effort, building up our basic understanding of the natural world can yield these results that are really informative.”