The Hoatzin is such a bizarre and unique bird that it almost has to be seen to be believed. Fortunately, seeing the Hoatzin is not difficult, as it is widespread in the lowlands of northern and central South America, and is fairly common throughout much of this region. Hoatzins live in trees and shrubs along the margins of lakes and slow-moving rivers. They often sit in groups in the open, although when disturbed they typically retreat into denser cover. They move awkwardly, however, and so give themselves away with sounds of crashing through the vegetation, accompanied by loud vocalizations. Hoatzins in effect are flying cows: their diet primarily is young leaves and buds, which are digested in the crop with the aid of bacteria and microbes. Hoatzins nest over the water. The young can swim, and so may drop to the water when threatened. The nestlings retain claws on their wing (lost in the adult), which they use in climbing back to the nest. The appearance of the Hoatzin is no less distinctive than their natural history, with their long, loose crest, bright blue facial skin, and red eye. For decades ornithologists have debated the relationships of this unique South American bird: at various times the Hoatzin has been thought to be related to Galliformes (cracids and relatives) or to Cuculiformes (cuckoos and relatives), but to date the Hoatzin has defied easy classification.