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.
Most scientists now support control only to restore local populations of threatened or endangered species. So the news is more good than bad.