"Habitat loss due to agricultural intensification and urbanization is arguably the biggest threat to birds, along with climate change," says Ashley Dayer, an associate professor in the College of Natural Resources and Environment and a Global Change Center affiliated faculty member at Virginia Tech. She also points to cats and window collisions playing a role in their deaths. Data shows up to one billion birds die each year after hitting windows. Cats are estimated to kill more than 2.4 billion birds annually in the U.S. and Canada.