One-fourth of all North American bird species are at risk, according to a new study released by the National Audubon Society. The report blames increased urbanization and the resulting loss of open spaces for the decline; as cities grow, farmlands are converted to urban areas and grasslands are converted to farmlands, leaving birds with insufficient habitat. The Audubon Society now has more than 200 birds on its “watch” list and says 21 are endangered — meaning that overall, nearly twice as many bird species are in trouble as were five years ago, the last time the report was issued. Frank Gill, the group’s chief ornithologist, called birds a primary indicator of environmental health and warned that bad news for birds is also bad news for humans: “We should listen to what their declines are telling us about the ecosystems we both inhabit.”