Story at a glance

  • A recent study found that North American bird populations have declined by 30 percent since 1970.
  • Now, a study of thousands of birds across multiple decades in Chicago finds that their bodies are getting smaller.
  • The researchers think climate change could be leading to the birds' decreased size.

The birds are shrinking, and climate change could have something to do with it. This is the startling finding of a new study of more than 70,000 birds across nearly four decades in Chicago. The migratory bird specimens whose bodies were measured for the study died following collisions with the Windy City’s glassy skyscrapers and were collected by the Field Museum of Natural History, the Washington Post reports

Since the museum started recording vitals for dead birds in Chicago in 1978, not only have birds’ legs gotten shorter, their bodies have gotten lighter and their wings have gotten a little longer. But why?

The authors of the research say that the warmer temperatures brought about by climate change appear to be behind the birds’ winnowing bodies. “Warming temperatures seem to be having a pretty consistent and almost universal effect on a large number of different species, regardless of other aspects of their biology,” study author Benjamin Winger told the Post.

The birds in the study were mostly made up of 15 common species including swamp sparrows and dark-eyed juncos, but rare birds such as the elusive yellow rail also made it into the collection. Between 1978 and 2016, the birds in the collection lost 2.6 percent of their body mass, their legs became 2.4 percent shorter and their wings got 1.3 percent longer. The changes are too small to see with the naked eye, but their appearance in such a comprehensive sample across species suggests a significant, widespread trend. 

Whether this trend towards more diminutive birds is harmful isn’t clear, and neither is the adaptive value of the changes. But when the researchers looked to environmental factors such as weather and plant growth for some kind of explanation, elevated temperatures in the summer most strongly correlated with smaller birds.

“In years when temperatures were a bit warmer, the bodies got smaller. In years when temperatures were a bit cooler, we saw an increase in body size, even though the long-term trend was to decline,” Winger told the Post. “So that leads us to believe that temperature is pretty important here.”

The researchers proposed some possible, but unconfirmed, explanations. Smaller bodies are better at dissipating heat, so higher temperatures might give individuals with less body mass a competitive advantage. Other research has also shown hot weather in breeding season reduces the size of zebra finch and house sparrow offspring. Finding wings getting longer over time was particularly surprising. Researchers speculate that extra length might make for slightly more efficient flight. 

While the trend could be benign adaptations to changing conditions, it has coincided with a 29 percent decline in North American bird populations since 1970.

Published on Dec 06, 2019