For nearly 40 years, a dedicated team of researchers and volunteers has been taking the measurements of thousands of birds that have collided with buildings in Chicago, Illinois—and they found a surprising trend. Birds across species have slowly been shrinking, The Washington Post reports. Researchers analyzed data from more than 70,000 individual birds collected between 1978 and 2016. On average, birds lost 2.6% of their mass, and had 2.4% shorter legs and 1.3% longer wings, the team reported Wednesday in Ecology Letters. The animals might be shrinking in order to stay cool in warmer temperatures, or their growth may be stunted by the heat, the researchers say. The longer wings are more mysterious, they said, but could serve to make flying more efficient.
*Correction, 16 December, 10:50 a.m.: The words “may be” have been added to the headline, as the link between bird size and global temperatures is a correlation.