Anatomy and Physiology of the Normal Avian Integumentary (Skin) System

Presenting Author: Susan E. Orosz, PhD, DVM, Dipl ABVP (Avian), Dipl ECAMS

Professor College of Veterinary Medicine, The University of Tennessee, Knoxville, TN

Interpretive Review and comments by Dr. Nemetz:

Dr. Nemetz finished a master's program in veterinary anatomy at UC Davis in 1981. A thorough understanding of normal anatomy is the foundation to understanding disease processes. All pathology (disease) alters the normal anatomy/physiology before clinical signs become evident so to understand pathology; a veterinarian must understand the "norm".  Dr. Orosz gave a fine review of the normal anatomy of the skin including the bill or beak, claws (nails), uropygial gland (oil gland) and the feathers along with the molting process.

The Avian Skin

The skin of a psittacine is generally thinner and more delicate than that of mammals. This is an important consideration in wound care, its susceptibility to damage, as well as in healing. It has poor attachment to underlying muscle (which makes it quite movable), but has extensive attachments to bones especially in the foot and hand (wing) area.

The Avian Beak

The horny portion of the "Bill" is called the "Beak" or rhamphotheca. This is a very specialized structure. It has growth centers along the rostral (forward) edge of the cere (flesh tissue around the nostrils) and in the lateral sides of the rhamphotheca. The differential growth of these growth centers creates the unique curvature to a bird's upper and lower beak. Trauma to the dermis layer is what causes abnormal growth. This statement is critical to understanding the pathology of the beak in mature birds. Trauma or diseases that affect dermal growth may cause an increase in the growth rate of the maxillary rhamphotheca. Undue pressure to one side of the beak in developing chicks, particularly in macaws, may cause the beak to curve (scissor beak) and Cockatoo chicks are more prone to formation of a longer lower beak (mandibular prognathism).

Malformations of the beak fall into THREE primary categories

1.        Lateral deviation of the upper beak (maxillary rhamphotheca), "scissor beak"

2.        Elongation of the lower beak (mandibular prognathism)

3.        Compression deformities of the mandibular rhamphotheca

Dr. Nemetz in conjunction with his father, a dentist, has worked on improving and correcting beak abnormalities since 1992. The key is to address the problems as soon as they are discovered, even when the patient is a very young chick. DO NOT BELIEVE ANYONE THAT STATES IT IS BETTER TO WAIT. The older a bird gets the more difficult and expensive a procedure for correction becomes and the more painful it is for the bird. Dr. Nemetz has corrected deformities in chicks as young as 3 weeks. Do not wait!

The Avian Claw or Nail

The nails are composed of a dorsal (top) plate, two lateral walls, and a softer ventral (bottom) plate. The faster growth of the dorsal growth centers compared with the ventral centers result in the curving of the claw. Injure to any of these centers will result in variable presentations of abnormal nail growth; laterally or dorsally (upwards).

The Avian Foot Pads

These are thickenings of the skin on the ground surface of the foot. They are designed to withstand compression. Damage to this area can lead to bacterial infections and "bumblefoot". Bumblefoot is a descriptive term to mean a swelling that appears like the abdomen of a bumblebee and does not give any indication to a specific etiology.  It is important that proper perch sizes be used for any particular bird to minimize excessive wear to the foot pad. Dr. Nemetz along with a veterinary student performed a study to determine an appropriate perch diameter for any given individual bird. Like humans, it was discovered that ALL birds have variable foot size (length from the tip of the most cranial toe to the tip of the most caudal toe = "Foot-length"). This was true even within a given species (i.e. macaws, cockatoos, parakeets). For example, if you own a cockatiel, not all cockatiels require or should have the same perch size. After measuring hundreds of bird's foot-lengths, it was determined that the optimal perch would be one where the foot-length covers 65-75% of the circumference of the perch. Multiple perches of varying circumferences are recommended, however for the primary perch, this calculation is recommended.

The Uropygial Gland (Oil Gland)

There are no sweat glands in the skin of birds. This anatomic fact answers a commonly asked question "Do our pet birds sweat?" The answer is NO! The only true glands of the integument in birds include the uropygial gland, sebaceous glands of the ear canal that secrete a waxy material, and glands of the vent that secrete mucus. The uropygial gland is not found in all avian species. It is commonly found in the budgerigar, cockatiel, African grey parrot, cockatoo, and macaws.

The uropygial gland has two lobes that have ducts that converge to form a median papilla. The lipid material secreted by the gland is spread over the feathers while preening. The lipid material contains the inactive form of Vitamin D which is activated by sunlight prior to ingestion during preening. This is one reason why birds should be exposed to the UV rays of the sun at least four hours each week. Window glass reflects a large percentage of the UV rays thereby minimizing the effects of birds placed in front of these structures. This gland can become impacted, infected, or become cancerous (squamous cell carcinomas, adenocarcinoma, and fibrosarcomas) and if not removed early, these tumors can invade the pelvic girdle.

Dr. Nemetz in 2003 has been working with a radiation imaging specialist to find a treatment for various cancers of the uropygial gland. Two cases to date have been treated using Strontium radiation with excellent success and no recurrence after several months. The key is to discover these tumors early. Some symptoms are excessive preening of the gland, bleeding in the tail head area, feather picking in this area, or excessive lipid (greasy feathers) production.

Avian Feathers

No living creatures grow feathers except for birds. Feathers are light weight and provide insulation, warmth, and reduce turbulence during flight. There are 7 types of feathers in birds. Feathers are arranged on the body in tracts as well as non-feathered areas (apterylae). These non-feathered areas are useful for pulling blood, placement of fluids, and for evaluating feather disease. Feathers are well attached to the underlying dermis or bone and the follicles have many free nerve endings with several pain receptors nearby. This is why feather extraction should ONLY be performed if necessary for uncontrollable bleeding, life threatening conditions, or under anesthesia in preparation for surgery. Feathers should not be extracted just because they look old or to initiate new feather growth in the case of damaged feathers.

Feather color is mainly from reflection of white light on feather pigments or unique structural configuration in the feathers. Nutritional deficiencies, stress, and thyroid hormone levels can alter the color of feathers. Stress bars, unkempt feathers, loose or ruffled feathers, or poor "zippering" of the feather barbs are all indicative of internal pathology.

A developing feather has a follicle made up of a central dermal core containing an axial artery and vein that retracts when the feather matures. These are called the "Blood Feathers" and if damaged can hemorrhage. The process of inappropriate wing clipping is the most common cause of damaged blood feathers and should be performed only by experienced persons. ( Click here to download The BIRD C News "Recommended Safe Method for Wing Clipping") If a blood feather is damaged, a powdered cautery agent should be applied to control the bleeding. DO NOT attempt to extract the feather as the axial artery may continue to bleed at the follicle's base requiring surgical intervention to successfully control the bleeding.

Molting of Feathers

Molting is "The shedding and renewal of the plumage." New erupting feathers are termed "Pin feathers" because they appear as pins erupting from the skin. Most species replace their feathers once a year, but it may be as often as 3 times a year. In California with poor distinct seasons, most bird species molt gradually throughout the year. In psittacine birds the presence of non-feathered patches is abnormal, not molting, and should be evaluated by an avian veterinarian.

The molt is controlled by a number of complex interactions including neurohormonal factors and environmental influences such as photoperiods. It also appears that thyroid hormones are not involved in the molting process, but are essential for growth and differentiation of the feather and the bird's pattern of feathering.

Conclusion: