A dozen dead Barn and Violet-green Swallows huddled together on the dusty desert floor of southern New Mexico. Numerous Western Bluebirds packed into a crevice in southern Colorado as if they panicked. Sparrows, lined up almost wing-to-wing, lying limply along the banks of the Rio Grande. These are just a few of the grisly discoveries recently made in what is likely a mass death event for migratory birds occurring across the Southwest. At the moment, there is no clear explanation. The die-off is “unprecedented,” says Martha Desmond, an avian ecologist at New Mexico State University (NMSU) in Las Cruces, who is leading the research team documenting the event. She estimates that hundreds of thousands and possibly even up to a million birds have died across at least five U.S. states and in four Mexican states. “It’s enormous, the extent of this,” Desmond says. “We haven’t counted all the species yet, but there are lots of species involved.” Online reports...