Disappearance of birds in North America

At one time, passenger pigeon flocks crowded the skies over the U.S. like clouds. Over 100 years ago, the pigeons were hunted out of existence. Now, a new study shows North American bird populations have decreased by 29% since 1970, almost 3 billion fewer birds. David Pearson, a research professor in the Arizona State University School of Life Sciences explains what’s happening to birds.