For each species of songbird whose population is on the rise in the U.S., two species are in decline, says Jeff Wells of the National Audubon Society. Of the 116 species whose populations have fluctuated since 1966, 76 have decreased significantly. Urban sprawl, and all that comes along with it (farmland and forest loss, more roads, etc.), is one of the big contributors to the loss in numbers. The American Bird Conservancy estimates that cats also kill hundreds of millions of songbirds a year.