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.
Order Sphenisciformes.-Nidicolous, marine.
Steganopodes.- Well flying, aquatic, nidicolous; with all the four toes webbed together.
Ardeae.-Piscivorous, nidicolous, waders; with complicated hypotarsus and with long cervical apteria.
Ciconiae.-Zoophagous, nidicolous, waders; with simple hypotarsus and without cervical apteria.
Carnivorous, desmognathous, nidicolous, without functional caeca.
Order Cuculiformes.-Desmognathous, nidicolous; zygodactylous, or with the outer toe reversible.