Major Anatomical and Physiological Differences between Birds and Mammals
Atlantic Coast Veterinary Conference 2001
Teresa L. Lightfoot D.V.M., Diplomate ABVP - Avian

We'll start out with some bullet points that distinguish avian from mammalian species. This should outline significant anatomical differences in birds, with particular reference to our pet birds. We will then discuss in more detail some of the organic and structural idiosyncrasies and how they relate to behavior, health, nutrition, and disease in pet and aviary birds.

Unlike most mammals, birds have:

1)  No diaphragm, so the abdomen communicates directly with the thorax including the lungs, heart and air sacs.

2)  A varying number of pneumatic (air-filled) bones that also communicate with the abdomen and thorax.

3)  An extensive air sac system that extends throughout the body with a complicated one- way airflow. This requires two cycles of breathing for air to be exhaled back out into the environment (a good reason birds shouldn't smoke - they can't claim that they never inhaled).

4)  A high metabolic rate, so the stress and subsequent increased oxygen demand of sick birds can become critical. Therefore, veterinarians and owners must consider the risk/benefit ratio of any diagnostic or therapeutic procedures prior to proceeding. There is no humor in this statement - this is one of the cornerstones of avian medicine. Even more than "First, Do No Harm", we must consider "First, Do Nothing" until the bird has been placed in an oxygenated environment and if possible, been allowed to equilibrate and stabilize to the best of its ability.

5)  Complete cartilaginous tracheal rings. One can not "strangle" a bird, (there is no soft spot in the trachea, called the tracheal ligament in mammals, that can collapse) however;

6)  Respiration is accomplished by free movement of the chest and abdomen - (very little internal lung movement occurs). Therefore, over-constriction of the body during restraint can cause oxygen deprivation.

7)  Kidneys that contain both mammal and reptile nephrons. The reptilian nephron contains no Loop of Henle. Therefore, Loop diuretics such as furosemide, do not work on these nephrons, but there are sufficient mammalian nephrons that diuresis can be accomplished with these diuretics.

8)  The end product of protein metabolism is uric acid, not urea. (This is why generalized visceral and articular gout are so prevalent in birds). The blood tests for kidney function are subsequently limited and less accurate than in mammals. Uric acid is secreted by the reptilian nephrons of the bird's kidneys. Dehydration does not generally cause elevations of uric acid levels. Elevation of uric acid in the serum usually indicates kidney problems. A urinalysis is often needed to initially diagnosis kidney problems at an early stage. Casts are a significant finding in an avian U.A. A subsequent kidney biopsy may be required to determine the type of kidney disease, prognosis, and proper treatment.

9)  A renal shunt, where venous blood circulating in the lower gastrointestinal tract and the caudal extremities may pass directly through the kidneys prior to being filtered by the liver. This is the reason septicemia (infection in the bloodstream) is common. To help correct this, birds will drink more water, and produce both more urine and watery stools with many infections. This often appears to the owner as a diarrhea. It is also the reason that injections are usually not given in the lower half of the body, since the medication may be excreted in part prior to achieving full effect in the blood stream.

10)  Both a proventriculus (true stomach) and ventriculus (thick walled grinding and heavy material storage chamber, commonly called the gizzard) in psittacine and passerine birds, among others.

11)  Most female birds have only one functional ovary (the left).

12)  The ability in female birds to store extra calcium in their normally hollow long bones in preparation for egg-laying. (and we have to drink milk and eat TumsR.....)

13)  An extremely long and flexible cervical spine. Instead of the average of seven cervical vertebrae as in mammals, birds have between 11 and 25. This is demonstrated by how well they can turn their heads and reach with their beak to preen almost any part of their body. They would be awesome in yoga class.. Conversely, their lumbar and sacral spines are much more rigid than in mammals. This provides more stability for the body and the extensive flight muscles.

14)  The ability to fly (a great advantage) and the inability to move in a coordinated manner on the ground (not such an advantage, but fun to watch)....

15)  The ability to imitate human voices, and other human /technology created sounds. This allows us, particularly with African Grays, to answer the bird instead of the phone, check the clothes drier when the timer did not go off, ditto the microwave, beeper, stove timer, door bell and alarm clock.

16)  A complete sternum and keel bone ridge, called the carina, without the intervening pronounced spaces that mammals have between their ribs. Attached to this rigid keel are the strong pectoral muscles that allow the down stroke of flight. In a captive environment with limited exercise and incorrect nutrition and/or excessive calories, this is a storage area for fat, where Amazons in particular are known to develop unwanted cleavage.

17)  Prokinesis, or the ability to move the upper beak forward independent of the lower beak. Rather than one tempomandibular joint, they have a quadrate bone, with several elastic attachments, that gives them basically a "double hinge" to their jaw. Needless to say, the pounds per square inch of pressure that can be applied to perches, earlobes or nostrils with this design is quite high.

18)  No teeth - so no dental caries, gum recession, nor embarrassing stains.

19)  Only one true sinus, called the infraorbital sinus, although in birds such as Amazons, Macaws and African Grays infection in this sinus can be quite severe.

20) No epiglottis (nothing to cover and protect the tracheal opening). This makes aspiration of food more likely and produces the great expulsion of air that accompanies a bird scream.

21) A locking mechanism that allows them to sleep with their feet perched and stable without conscious effort (can you imagine our dogs, cats, or us for that matter, sleeping standing up night after night)? Some lutino cockatiels may be partially lacking this ability; among additional congenial or genetic problems inherent is some members of this mutation cockatiel.

22) Scales, reminiscent of reptiles, on the legs and feet.

23) Various types of feathers, performing, besides the ability to fly, functions such as insulation from the cold and from the rain, acting as sensory receptors, producing powder down, and protecting structures such as the eye.

24) A choanal slit. This is the natural opening between the nares/sinuses, and the roof of the mouth. If present in a mammal, it would resemble a cleft palate.

25) A limited lymphatic system without true lymph nodes as are present in mammals

26) A spleen that doesn't store blood. It does produce white blood cells and destroy old red blood cells.

27) The ability to lay eggs, whether infertile (comparable to ovulation in mammals, but potentially causing much more problems for birds in captivity) or fertile.

28) Numerous variations in their hormone types, locations and functions. For example, diabetes mellitus is not uncommon in birds, especially cockatiels. As in most psittacines, it is not a lack of insulin production that causes diabetes, but rather an increase in the amount of glucagon (the opposite of insulin). Therefore, regular insulin does not work well to control diabetic psittacines.

29) The ability of bones to heal rapidly, especially in young birds. Prior to any changes associated with healing being visible on an X-ray, a fibrous callus forms around the broken bone and gives it excellent stability.

30) Retain a "prey' species defense status. This causes birds to disguise signs of illness as long as possible in order to avoid attack by predators. Therefore, the most minor signs recognized by an owner, such as decreased talking or activity, are often life saving when recognized and presented to the veterinarian for treatment prior to more serious debilitation.

31) The ability to make more noise, at the resonant frequency of the owner's sinuses per cm2 of body surface, than any other species (expect maybe an angry sugar glider).

Now, let's discuss a few anatomical points that may have associated misconceptions or incomplete information.


The beak of psittacines birds is a living structure. The outer most portion is hardened and has no nerve endings. But just below this surface are blood vessels, sensitive nerve endings, and a hollow area that is the extension of the infraorbital sinus. The beak grows not only in a downward direction, but also an outward manner. The growth plate (site of the new cell produced for the beak) of the maxilla is located at the attachment to the cere. Separation or severe damage at this point is difficult and sometimes impossible to repair. Unfortunately, this is often where injuries, especially in fights with other birds, can occur. The lower beak (mandible) has its growth originating at the junction with the skin of the lower jaw. The same tragic effects can occur when extensive tearing damage is done to the lower jaw.

The outer, hardened portion of the beak (called the rhinotheca on top and the gnathotheca on the bottom) is composed of keratin and various crystals. Abnormalities of beak growth are often a reflection of poor nutrition and or accompanying liver disease. Adequate nutrition is needed for the development of proper beak tissue, and a fully functional liver (which is also dependent on good nutrition) is needed to convert this nutrition into useable material for the beak, skin, and feathers. This point will be returned to repeatedly. Nutrition and husbandry are the underlying causes of most non-contagious but nevertheless potentially fatal diseases in our pet birds. The earlier we start our birds on the best appropriate diets, the better their organ development will be, and this will subsequently result in longer and higher quality anticipated life spans. This is a good place to discuss nutrition.


No one has the complete answer to balanced diets for all of the different psittacine species. However, we have learned a considerable amount in the past two decades. Obviously, we learned (myself along with everyone else who was breeding birds in the 1970's) that sunflower seed diets alone are not healthy. We have advanced greatly, but still maintain some misconceptions. First, as a veterinarian or veterinary technician, what one will see is very well-intentioned owners feeding or at least offering a wide variety of table foods. This may be acceptable, IF one is assuring a balance of table food that is: 1) Nutritionally sound. This usually means organically grown, and therefore not devoid of many of the nutrients nor loaded with pesticides and preservatives. 2) Taking into account the less carnivorous and more herbivorous and fruitivorous nature of our birds. 3) Assuring that the birds actually consume, in proper portions, the table foods that they are offered. Although pelleted diets may not be a panacea, they are a tremendous improvement over our mixed seed diets as a base, especially when started in young birds. The fewer dyes, flavorings and preservatives that are added, the better for our birds. Beware of products that look, smell and taste good to the owner. The general content may still be reasonable, but these additives may be detrimental, and are often added for appeal to the owner, not the bird. My personal belief is that most pet birds need some supplementation, even to the best organic pelleted diet, not for nutritional but for psychological reasons. Sticking with dark green, leafy vegetables, with some pasta, rice, or other non-salt containing carbohydrate and occasional seeds (especially for breeding) can round out the bird's psychological needs, as well as stimulate breeding when desired. In a pet bird situation, you may NOT want to include this seed and excessive greens component, as it may lead to unwanted hormonal activity (such as biting, screaming and egg laying). In any case, it should still be a minimal percentage of the total daily food intake.


In additional to the increased oxygen requirements of birds mentioned previously, several other factors of avian respiration are important in the prevention, diagnosis and treatment of disease in birds. Since there is no separation between the chest and abdomen, masses, fluid or other abdominal disease in birds can make respiration more difficult. Birds will often appear to have respiratory problems, exhibited by tail bobbing, open- mouth breathing and increased respiratory rates, when in actuality there is an infection or a mass in the abdomen that is causing this increased respiratory effort. Also, the communication of air into the bones of birds makes it possible for a fractured bone to be the cause of respiratory distress or infection. A physical examination will determine if there is increased space between the end of the keel bone and the pubis (generally indicating an abdominal mass). Blood work and radiographs are useful in determining if abdominal organ enlargement exists that is causing the respiratory problem. Care must be taken in obtaining these tests, since once again, a bird with respiratory compromise has very little oxygen reserve when stressed. A contrast study or barium X-ray will not only demonstrate the intestinal tract (although that is where the barium will be located), it also contrasts with the other tissues on the radiograph, allowing the identification of areas of increased or abnormal density (i.e. kidney, gonad, liver).

Diarrhea VS. Increased Urination

Since birds possess a cloaca, the urinary and fecal components are stored and delivered from the same orifice. This makes it easy to mistake an increase in urine production for diarrhea. When a bird has a condition such as diabetes mellitus or kidney disease, it will often have a pronounced increase in urine output. Over-the-counter treatments for "diarrhea" abound. Some people report that their bird was temporarily better after receiving one of these medications, which are often administered in the drinking water. May of these medications contain a mild sulfa-based antibiotic, and the taste of this may decrease the bird's water consumption, thereby decreasing the liquid excreted in the urine and stool. As the urine mixes in the cloaca with the stool, the fecal portion will become somewhat loose. When the dropping is released, the stool may look like a diarrhea. However, there is generally a much more pronounced area of pure liquid around the dropping, indicating that the primary problem is increased urine, not diarrhea.

Conditions do exist that produce true diarrhea. Infections or other diseases of the liver, parasitic conditions, toxicities, bacterial, yeast, or fungal infections of the intestinal tract, systemic viruses - all of these may produce diarrhea. Some of the same blood tests that are needed to diagnose the cause of increased urination will also show the affected organ or organs in cases of diarrhea. A fecal gram stain, although a useful tool for diagnosing simple intestinal infections, and also for assessing the general health of the bird's digestive system and dietary balance, will not reveal any information concerning other causes of diarrhea, nor any of the causes of increased urination. If limited to performing only a gram stain, then the diagnostic results will also be limited to the few conditions that this procedure can detect. Unfortunately, many times birds have been misdiagnosed and treated for diarrhea when the actual problem is an increase in urine. As noted earlier, the most significant finding on a microscopic examination of the urine is casts. The production of casts in birds indicates renal compromise.

As a prey species, in the strange environment of the animal hospital, a bird that is sick may mask clinical signs, attempting to act normal to hide its illness. The owner's awareness and reporting of changes, such as decreased talking, decreased appetite or increased sleeping are vital indications that their pet bird is truly sick. Early detection and treatment of disease are critical in birds and these start with the observations of the owner.

(Portions reprinted with the permission of EVS, Inc., 1999) Recommended reading list is found in the section following Chronic Egg-Laying and Egg Bound Birds.

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Teresa L Lightfoot