Section II—External Anatomy
The bill (or beak) is the most prominent feature of the head (Figure 8.7). It includes the maxilla and mandible, which are covered by a horny sheath or rhamphotheca. Open the mouth and note the absence of teeth.
The nares (sing., naris) pierce the maxilla. Immediately posterior to them is the operculum, a soft swelling of the integument. The eyes are large, as would be expected in a vertebrate that depends largely on vision. The external acoustic meatus, leading to the tympanic membrane of the ear, lies posterior and just ventral to the level of the eye, but is concealed by feathers (Figures 8.8, 8.9).The most obvious feature of birds is the presence of feathers, which in most birds serve primarily for flight and temperature control and will be examined in more detail below. The neck appears shorter than it really is (see Section I), but this is due to the covering of feathers. The forelimbs are strongly modified to form wings. Spread the wings and note the prominence of the feathers. In contrast, the distal part of the hind limb, including the pes, is covered by horny scales. branch out from each barb and each barbule bears tiny hooklets that interlock with hooklets of adjacent bar- bules to help maintain the shape and structure of a feather. Magnification is required to observe barbules and hooklets. The barbs are arranged in two vanes. The vanes are usually symmetrical around the rhachis in most contour feathers covering the body, but are asymmetrical in most of the flight and, to a lesser degree, tail feathers. In flight feathers, the vane facing the leading edge of the wing is narrower than at the trailing edge.
Feathers are designated by their position on the body, and there are numerous different types. Here, only the more typical ones important in flight on the tail (Figure 8.7) and wing (Figure 8.9) are distinguished.
The remiges (sing., remex) are the large feathers of the wing. On the trailing edge of the wing, the primary remiges (or, simply, primaries) are the feathers attaching to the distal end of the wing, the carpometacarpus and phalanges, and are numbered from 1 to 10 in proximal to distal order. Those attaching to the ulna are the secondary remiges, of which there are usually 15, numbered in distal to proximal order. The tertiary remiges, usually four in number, lie proximal to the most proximal secondaries. The coverts, which vary in size, cover nearly all of the remainder of the wing. The exception is a small patch of (usually four) alular quills, which arise from the alular phalanx and together form the alula or bastard wing. Though relatively small, the alula is an extremely important flight structure that helps avoid stalling during low velocity flight. The feathers of the tail are termed rectrices (sing., rectix).Key Terms: External Anatomy
The dorsal region is the dorsum, whereas the ventral region includes the expanded breast anteriorly and the venter or belly posterior to it. At the posterior end of the body, the tail includes the short posterior, fleshy stump of the body, the uropygium, as well as the feathers extending from it. Spread the feathers on the dorsal surface of the uropygium to reveal the uropygial gland (Figure 8.11). The gland produces an oily secretion used in preening that protects the feathers. Spread the ventral tail feathers and note the slit-like opening of the cloaca (Figure 8.14).
There are several types of feathers in birds, but only the larger contour feathers are described here. Contour feathers are those that are typically thought of as feathers, such as the flight feathers on the wings and tail, as well as smaller feathers that cover much of the body. Examine a contour feather (Figure 8.8) from the wing of the pigeon. Note the central shaft or quill. The calamus is the hollow, basal part of the quill, while the rhachis is the part bearing barbs, the tiny, parallel structures that branch from the rhachis.
Numerous barbules| alula (bastard wing) | neck |
| alular quills | operculum |
| barbs | primary remiges |
| barbules | rectrices (sing., rectix) |
| bill (beak) | remiges (sing., remex) |
| breast | rhachis |
| calamus | rhamphotheca |
| cloaca | scales |
| contour feathers | secondary remiges |
| coverts | shaft |
| dorsum | tail |
| external acoustic meatus | tertiary remiges |
| eyes | uropygial gland |
| feathers | uropygium |
| forelimbs | vanes |
| hooklets | venter |
| nares (sing., naris) |
FIGURE 8.7 External features of the pigeon, with forearm (wing) abducted, in left lateral view.
FIGURE 8.8 Feather of the pigeon, with blowups showing successively finer detail of structures.