I observed a Red-winged Blackbird (Agelaius phoeniceus) nest in southcentral Wisconsin, USA that received a Yellow-billed Cuckoo (Coccyzus americanus) egg. The cuckoo egg was laid after the blackbird clutch was complete and the female had begun incubation. The parasitic egg was considerably different in size and color, but was accepted by the female Red-winged Blackbird. The female, but not the male, also cared for the nestling, which was strikingly different in plumage, gape markings, and begging behavior, and which grew despite its 3–4 day hatching delay relative to its nest mates. Rapid development and exaggerated begging behavior of the Yellow-billed Cuckoo nestling may enable a parasitic cuckoo nestling to compete successfully with host nestlings for food.