Cowbird eggs began appearing in the nests of many more species, alarming conservationists and birders. One concern was that the eastern songbirds, completely unfamiliar with the cowbird's technique, might dwindle in number. And indeed, some songbird populations have been declining. There was special concern about species that were already rare, like Kirkland's warbler, found in nesting season in very limited regions of northern Michigan. According to the National Audubon Society, cowbirds have been known to parasitize more than 220 host species.
Some species detect the invading egg and dispose of it.
But more than 150 species have successfully reared the cowbird chicks. Because they feared a threat to songbirds, many federal and local government agencies began cowbird control programs a quarter century ago. Cowbird parasitism is probably not responsible for declines of many North American songbird species, and its effect has been overestimated by experts. Host species often renest and make up for the lost brood. Cowbird control is very expensive and somewhat controversial.