Slowly, steadily and almost imperceptibly, North America’s bird population is dwindling.

The sparrows and finches that visit backyard feeders number fewer each year. The flutelike song of the western meadowlark — the official bird of six U.S. states — is growing more rare. The continent has lost nearly 3 billion birds representing hundreds of species over the past five decades, in an enormous loss that signals an “overlooked biodiversity crisis,” according to a study from top ornithologists and government agencies.

This is not an extinction crisis — yet. It is a more insidious decline in abundance as humans dramatically alter the landscape: There are 29 percent fewer birds in the United States and Canada today than in 1970, the study concludes. Grassland species have been hardest hit, probably because of agricultural intensification that has engulfed habitats and spread pesticides that kill the insects many birds eat. But the victims include warblers, thrushes, swallows and other familiar birds.