Birds and Birding's Guide to:

Watching THE ANATOMY OF BIRDS

Bird Perching Bones

The upper end of the tarsus and its relation to the tendons is a fair index to the position of its owner, being simplest in Ostriches and other birds undeniably low or generalized in character, and more complicated in higher forms, reaching its greatest complexity in the perchers, in which the hypotarsus, as it is termed, is pierced for the passage of four or five tendons.

In the Crow, for example, there are four large and one small perforation, in the Clamatores but four, and one of these is closed by cartilage and not by bone; in the Picariaa there are but one or two tendinal perforations, and in the divers but one, so that the specialization of the tendons and that of the tarsus go together.

The three divisions of the lower end of the tarsus indicate that it is composed of three bones, but these bones are clearly shown only in embryos, in young Ostriches and in Penguins, where the bones, though united, are plainly indicated throughout life, this retaining of a primitive condition being one of the characters which has led many good authorities to place the Penguins in a group contrasting with all other fan-tailed birds.

The leg attaches to the hip-bone, or pelvis, each half of which is composed of the usual three bones, ilium, ischium, and pubis, although these fuse together at an early date and show as separate bones only in very young birds.

In all birds the pubis is directed backwards, and the greater part of the ilium lies in front of the hip-joint, this being a point where existing birds differ from existing reptiles. In the lower birds, as the Ostriches and Tinamous, the two principal bones of the pelvis, the ilium and ischium, are free from one another behind, this being a primitive character in which these birds resemble reptiles. The Cassowaries are an exception in having these bones united.

 

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