Cowbird mothers keep watch on the nests where they've laid their eggs.
If the birds find that their eggs have been destroyed or removed from the nest, the cowbirds retaliate, the study says.
The birds reportedly destroy the host birds' eggs, pecking holes in them or carrying them out of the nest and dropping them on the ground." source
This is part of an ongoing personal project where I'm working my way through illustrating the birds my wife and I have seen together. Slowly but surely I'm chugging my way through some of my favorite North American species.