Chapter 36 Feather Follicle Extirpation
Operative Techniques to Prevent Zoo Birds from Flying
In zoos, birds such as flamingos, pelicans, and waterfowl are ideally exhibited in large areas, often with lakes and islands. In such displays, the birds have to be prevented from escaping. This may be achieved by constructing large free-flight aviaries or by applying different flight restraint methods to the birds. Preventing birds from flying is an important ethical issue and has been discussed extensively, although without satisfying conclusions so far.
However, the method of feather follicle extirpation could be applied to certain bird species without inappropriately influencing their normal behavior because swimming, diving, walking, eating, and maintaining balance during copulation are still possible. Obviously, the problem of maintaining balance is not as challenging for follicle-extirpated birds as it is, for example, for pinioned birds9 (Box 36-1).
The skeleton of the wing consists of the humerus, ulna, radius, ossa carpi, and ossa metacarpalia (carpometacarpus) plus three fingers (phalanx proximalis minoris, phalanx proximalis digiti majoris, phalanx distalis digiti majoris; Fig. 36-1). The thumb phalanx, digiti alularis, is located at the os metacarpale majus.
In normal position, with the wing folded, the humerus is resting at the body. Two major muscles are inserting at the humeral bone. The Musculus biceps brachii is responsible for flexion and the M. triceps brachii for the extension of the wing. The pneumatized humeral bone is supplied with air via the saccus clavicularis.
In birds, the ulna and radius run parallel to each other without rotational movement and the ulna is larger than the radius. The carpalia and metacarpalia are joined and reduced to a few bones, such as the carpometacarpus, os carpis ulnare, and os carpi radiale. There are three digits; the thumb (alular digit) may have one or two phalanges and is called the alula. The alula has an important aerodynamic function by capturing the uplifting wind. The second finger (digitis major) has two phalanges. The third finger digit is a minor digit, with only one phalanx. The joints of the wing may only be flexed and extended to obtain a rigid wing. During flight, the main force comes from the distal wing, whereas the proximal region of the wing is providing the uplift.
Feathers have a cornified epidermis. During growth, the feather is supplied with blood from the follicle. The venous and arterial vessels degenerate once the growth is completed. Therefore, traumatization of newly growing feathers can cause severe blood loss.
The pinion is a contour feather with a hollow shaft and a vane. The shaft consists of the big shaft (rhachis) and the calamus. The vane has plenty of rigid filaments (barbae) in a fan-shaped position. Those carry additional finer filaments (barbulae), which are interconnected with little hooks (hamulus). The umbilicus proximalis is situated at the end of the quill (Fig. 36-2).1,10
The follicle is a tubular invagination of the epidermis with a papilla of corium at its base, which reaches into the umbilicus of the feather and is equipped with a system of ample blood vessels. The outer follicle sheath is formed by corium and is the base for the feather muscles, Musculi penarum. The inner feather sheath is lined with the epidermis and consists of squamous cells, some dead and some alive.
At the point of the calamus is a circular opening, the lower navel umbilicus proximalis inferior, where the corium reaches into the calamus and forms the corium papilla, covered by epidermal cells. This is the area of transition from the live cells of the follicle to the dead epidermal cells of the calamus. Once the growth of the feather is completed, the germinal activity rests until the next regenerative cycle. During the life of a bird, each feather papilla continues to produce feathers.1,5,10
The feathers of a wing are arranged in an overlapping position, anchored by an epidermal collar. This provides a solid but light surface, with an aerodynamic shape. Distal to the carpal joint, the remiges primarii 1 to 6 are attached at the metacarpus and 7 to 11 at the phalanxes. The 11th flight feather is often quite small, attached at the second phalanx of the second digit, called the ranicle.
The number of the secondary flight feathers at the arm (remiges secundarii) varies more and increases depending on the length of the ulna. Hummingbirds and most sparrows have only 10 and albatrosses have 38 to 40.1
Unlike the flight feather at the hand, the secondary flight feathers are counted from the carpal joint toward the proximal wing. The spaces between the vaneless quills of the flight feathers are covered by several rows of body feathers (tectrices). The larger body feathers at the carpal area insert distally to the flight feather, whereas the smaller tetrices of the proximal wing insert proximally to the secondary flight feathers. Like the second and third digits, the thumb carries flight and body feathers, which form the alula and influence the flight.1,10
The corium and epidermis are developing thick layers and develop an elevation at the corium papilla. The epidermis emerges as a quill from the epidermal collar. The sheath is formed by keratinized layers of epidermal tissue. Inside the skin, a feather follicle develops. Mostly active germ cells at its base form a tube, with the pulp inside. The pulp is filled with loose connective tissue, nerves, arteries, and veins. They degenerate during feather maturation and are resorbed later. The keratinized feather sheath dries from the top and breaks off; the vane is slowly released.
Out of the epidermis, cells develop into rows of spiral ledges, which change into keratin and form the rami. The feather shaft forms from parallel ledges. The rames grow out of the feather sheath. The feather unfolds completely and the pulp degenerates. As keratinization proceeds, a hollow shaft develops.1,5,10