The gray catbird is a poor foster parent, and that is precisely its advantage. When a brown-headed cowbird slips an egg into its nest, the catbird will often recognize the impostor and remove it, denying the intruder a meal ticket. Other songbirds are less discriminating. The cost can be steep.
The catbird's defense begins with a striking mismatch. Its own eggs are typically blue-green, while cowbird eggs are pale and speckled; egg recognition allows the host to respond to that difference rather than incubate whatever appears beneath it. A catbird can grasp an unwanted egg in its bill and carry it away. Many other hosts accept cowbird eggs, although the reasons vary: some may not reliably distinguish them, while others may lack an effective way to remove them without damaging their own clutch. Recognition is not infallible, either. A mistaken rejection wastes an egg the parent has already produced.
The conflict is better understood as brood parasitism than as a failure of parental care. Cowbirds leave incubation and feeding to hosts, and their chicks can compete with the host's young for food. Natural selection can favor rejection where the benefits exceed the risks, but different species face different pressures and constraints. The catbird has another talent. Its namesake mewing call is only one sound in a repertoire that includes whistles, squeaks and imitations of other birds. In the nest, its response is less elaborate: an alien egg may simply be carried out, leaving the blue-green clutch behind.
