So we have parasitic cowbirds. Here it is getting interesting, though, and proves Charles Darwin right. Some resident birds won't tolerate freeloaders. The Baltimore Oriole, for example, has had it with Affirmative Action. It will puncture a cowbird egg and just throw it out.
Robins do likewise.
Both robins and orioles are thriving. So what this means is that some songbird species are learning to assert themselves to their own benefit, increasing their survival chances. Here we have parasite young crowding out the legitimate young in the nest . . . Here we have parents spending their precious time and energy feeding and caring for the parasite young. He is so right.