Birds and Birding's Guide to:

Watching THE ANATOMY OF BIRDS

THERE are, broadly speaking, two sets of characters to be found in the skeleton of any animal. There are, first, those which bear a direct relation to its position in the class of vertebrates, bear witness to its origin and relationship, and are shared to a greater or less extent by every member of the group to which it belongs. These are termed structural or morphological characters.

There is also another set of characters which are connected with the animal's habits or mode of living and are believed to have been acquired during the development of the species, and are called secondary or teleological. It is owing to the fact that the skeleton is influenced by these two great factors that it is possible to tell from the skeleton, or even from parts of it, not only what position the animal holds in the scale of life, but what were its habits as well. So the modifications of a bird's skeleton are primarily morphological, those due to the fact that it is a bird, but these are associated with others rendered necessary by the adaptation of the fore limb for flight and its consequent withdrawal from use in any other form of locomotion, or for taking food. And in birds these last so overlay the others that the classification of birds is a very difficult matter. This is well shown by the long use of the divisions Ratitae and Carinatse, which depended mainly on the presence or absence of a keel to the sternum, when this and a number of other characters depended upon whether a bird did or did not fly. So the structure of a bird's skeleton depends first upon the fact that it is a bird, secondarily upon the manner in which it moves about, and thirdly, to a still less degree, upon the way it gets its living. Flight in itself is not a distinctive character, for mammals fly to-day, and reptiles, in the shape of pterodactyls, flew ages ago, having successfully mastered the problem of flight about the time the bird had taken its first lessons in the art. But the manner in which the fore limb is modified, the ground plan, so to speak, on which it is built, is a primary morphological character and differs in the birds, mammals, and reptiles.

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