Of course this doubleheaded condition is the more primitive, and as such exists in most nidifugous birds, but in many of these, as well as in many nidicolous birds, either the caudal or the iliac head is absent, and in a very few (Cancroma, Dicholophus, Steatornis and some Cathartes) the whole muscle is absent.
One species, the guacharo (Steatornis caripensis), or oil-bird, is commonly said to occur only in Venezuela, though it is found in Colombia and Ecuador also.
Species of the pheasant and partridge are not uncommon, and the " guacharo " (Steatornis caripensis), once believed to inhabit Venezuela only, is found in Ecuador also.
But one species of the genus Steatornis is known.