One male Gray Catbird was observed to be mated to two females in different territories. The territories were separated by another catbird territory, but the male defended both of them.
The male Gray Catbird uses his loud song to proclaim his territory. He uses a softer version of the song when near the nest or when a territorial intruder is nearby. The female may sing the quiet song back to the male.
Although the Brown-headed Cowbird lays eggs in Gray Catbird nests, the catbird throws most of them out. The catbird learns to recognize its own eggs, probably by looking at them. If a cowbird quickly replaces the first catbird egg in a nest, the catbird may recognize the cowbird egg as its own and throw out its own eggs as they are laid. But such mistakes are rare, and few catbirds are ever seen incubating cowbird eggs.