A new study has found that North American migratory birds have been getting smaller over the past 40 years. Researchers say the finding suggests a warming climate could be affecting bird growth in North America, and across the world. The study was recently published in Ecology Letters. It involved data on nearly 80,000 birds killed from 1978 to 2016 in the city of Chicago, Illinois. Researchers measured the size of the birds. The birds included in the study had died after crashing into buildings during the spring and fall migrations. Fifty-two species of birds were studied. Most were different kinds of songbirds. These birds reproduce in cold areas of North America and spend their winters in areas south of Chicago. Over the 40-year period, body size decreased in all 52 species.