Migratory birds in North America are shrinking as their wings get bigger. Climate change is to blame.

North American migratory birds are becoming smaller as the planet warms due to climate change, a new study finds.

Researchers from the University of California, Los Angeles (UCLA) examined more than 30 years of data for adult male birds across 105 avian species that migrate through North America. They found that between 1989 and 2018 the birds' body masses declined by about 0.6% on average, according to an Oct. 27 study in the journal Nature Ecology and Evolution.

The species that "experienced the greatest change over time" was the tree swallow (Tachycineta bicolor), said study lead author Casey Youngflesh, a quantitative ecologist from UCLA and a presidential postdoctoral fellow in the Ecology, Evolution, and Behavior (EEB) Program at Michigan State University. In this songbird, known for its striking iridescent blue feathers, body mass dropped by nearly 3%. Data used in this study came from the Monitoring Avian Productivity and Survivorship program (MAPS), part of the Institute for Bird Populations, a California-based nonprofit that studies bird population decline and has 1,200 bird banding stations throughout North America.

So, what's causing birds to shrink, especially over such a relatively short time period? Scientists suspect that climate change is the most likely culprit, and birds are adapting accordingly. 

"If you're a larger person and you're in a cold environment and let's say you have a very small friend, you will probably be able to withstand the cold a little bit better than a smaller person would," Youngflesh told Live Science. "This really comes down to surface area and volume. As a larger person, you'll be losing less heat than a smaller person. The same thing applies to birds."

Related: 10 of the biggest birds on Earth

In other words, smaller-bodied birds have a larger body-surface-area-to-volume ratio, so they need to expend less energy to keep cool. By comparison, birds with larger bodies are better equipped for conserving heat, according to the study.

Scientists also found that the size of a North American bird is largely dependent on where it resides — even for birds of the same species.

"We see that birds are getting smaller over time in response to temperature [change] and we're seeing the same thing over space," he said. "For example, a cardinal living in a really warm area of the United States is going to be generally smaller than a cardinal in a very cold climate, so there's a spatial effect [that's occurring]."

Conversely, while some bird species are decreasing in size, their wings aren't keeping pace, resulting in them having larger wings relative to their bodies. This is especially true for birds living at higher elevations.

"If you've ever spent time at [high] elevation, it's harder to breathe since there are literally fewer air particles, making the air thinner," Youngflesh said. For birds, thinner air at a higher elevation results in less lift. He pointed to helicopters as an example. “[There are] pilots who won't fly at very high altitudes because of this lack of reduced lift," he added.

Youngflesh is quick to note that this change in body size isn't happening across all North American migratory bird species.

"In some species, they aren't getting much smaller and the effect isn't as large as some other species," he said. "And that could be due to a number of factors that are important for the size and shape of birds," such as the elevation of their habitat.

"As you go up in elevation, say on a mountain, it's generally colder," Youngflesh said. "But birds are actually smaller there, and that has to do with the importance of flight."