Climate change is causing birds to shrink, says a new study conducted by researchers from the University of Michigan and the Field Museum in Chicago. Researchers analysed over 70,000 specimens from 52 North American migratory bird species collected over 40 years. They found that almost all of the species were getting smaller and their wingspans are growing. However, scientists aren't exactly sure why warmer temperatures cause birds to shrink. More studies may help understand how animals will adapt to climate change.