I saw this male at Burnaby Lake, my second time seeing these cowbirds after seeing some at Reifel a few days earlier. These birds are brood parasites; they do not build nests. Instead, the female finds a nest, usually belonging to a smaller species, and lays her eggs there. A few species recognize the cowbird eggs and dispose of them. But most species do not differentiate them and even feed the juvenile cowbirds. I learned about Brown-headed Cowbird behaviour from All About Birds. There is even a picture of a Common Yellowthroat adult feeding a much larger Brown-headed Cowbird juvenile.