I don't know if mockingbirds do this or not, but there are birds that do. Most birds have calls that are used as warnings, usually to signal that they themselves are dangerous. Some, however, use those warning calls to warn others of their species. If there's a birds who decides not to participate in warning of danger, the rest of the birds will punish it by not warning it when it's in danger. It's a simple but effective way of enforcing cooperation, and it's a pretty neat behavior.