Songbirds are shrinking and climate change may be to blame

Over the last 40 years, North America's songbirds have been getting smaller. 

Scientists made this disturbing discovery after analyzing a collection of more than 70,000 bird specimens across 52 species from Chicago's Field Museum.

All the specimens were gathered by Dave Willard, an ornithologist from the Field Museum who started the collection in  1978. He went out every morning before sunrise during spring and fall migration to collect fallen birds that crashed into nearby buildings while passing through Chicago. 

Willard originally collected them to contribute to the museum collection. But scientists soon realized the value of his collection, and decided to study the specimens to see if there were any long-term changes in North America's migratory bird populations over time. 

They found that the birds' body masses, lower leg bone lengths, and overall body size all decreased over time. Their leg bone length, which is an indicator for overall specimen size, decreased by 2.4 per cent across species. Meanwhile, the birds' wingspans increased by 1.3 per cent. 

Shrinking birds and rising temperatures 

"The thing that jumped out right away was the body size, which was obviously decreasing," said Dr. Brian Weeks, the lead researcher of the study.

"But the consistency of all the species getting smaller was very surprising." 

Weeks suspected that warmer temperature is behind the decline. It's not clear how it caused these birds to shrink, but one theory is that smaller animals tend to fare better in warmer climates because they lose heat more quickly than larger animals given their larger surface-area-to-volume ratios. 

Weeks also found strong evidence in the historical data in which a rapid increase in temperature over a period of time corresponded with a steep decrease in body size, while controlling for other factors. 

"It's hard to definitively say warm temperature led to a shrinkage in size," Weeks explained. "But based on observation, there's really strong evidence that supports that." 

He's concerned that this might have implications for the birds' ability to complete their seasonal migration, and has already seen an increase in the birds' wingspans to compensate for their smaller size in order to help them make the long flights each year. 

But it's not clear whether there might be limitations on how they'll be able to respond to the warmer climate in the future as global temperature continues to rise.