The cuckoo lays its egg in the nest of another bird where it is hatched and raised by a foster mother. The baby cuckoo is larger than the other chicks and usually kicks the biological babies out of the nest. The mother goes right on raising it.
I looked up pictures of cuckoo eggs and cowbird eggs, since the odd egg could belong to either. Both are "parasitic" layers. This egg more closely resembles that of the cowbird, so I'm going with thei cowbird theory.