Birds belong in a class all by themselves. This class is called “Aves”. Although there are over 8,600 species of birds, all of them, no matter what type, have certain basic characteristics and needs in common. Following are some general observations about the anatomy and physiology of all birds.
Birds have an extremely high metabolic rate. They must maintain a very high body temperature, which can be from 104 to 112 degrees. The average is 107 degrees. This high metabolic rate allows birds to generate the energy for flight. This energy also needs to be dissipated to keep the body temperature within the normal range. Both the respiratory tract and the feathers work to help regulate the body temperature.
The feathers can trap air, which provides an insulation effect. A bird exposed to cold temperatures or a bird that is sick will fluff its feathers and crouch, in an effort to conserve heat. Therefore, birds with wet feathers, birds undergoing molt or birds with feather loss due to picking or disease have a disadvantage in conserving heat. These birds should be kept at an ambient temperature of 80-90 degrees to prevent problems caused by heat loss.
Bird feathers derive from reptile scales, an evolutionary process. There are five different kinds of feathers. Flight feathers are the long feathers on the wings and tail. Contour feathers are found over most of the body. They are the most visible. They streamline, contour, cover, provide color, and insulate the body. Down feathers are the small, tufted feathers that are on very short stalks. They form the plumage of nestling and are found under the contour feathers in the adult. The filoplumes are hair-like, made of a fine shaft with small, tufted barbs at the tip. They are thought to monitor location. These feathers feel the drafts and detect wind changes while the bird is flying or moving through the air. Bursal feathers are the feathers around the face.
A few things to be known about feathers:
A feather will grow back in 2-4 weeks if you pull out the entire feather at the base. It will not grow back if you cut it off. Only normal molt will replace it. When surgery is to be performed, we prefer to pluck the feathers rather than clipping or shaving them, so they will grow back.
Feathers are arranged in rows or tracks (pterylae). You can brush the feathers away to one side and you will see areas completely devoid of feathers (apterylae). This is normal. The feather lays over what is an apparent “bald spot”.
Birds molt. This is, they go through a cycle of loss and replacement of feathers. Molting is greatly influenced by nutrition, age, sex, the season and the environment. With the environment, it is the photoperiod (how many hours of daylight and dark) that regulates the molt. With the season, most molt only their primary feathers once, secondary feathers twice during the year. Some birds molt continuously throughout the year but may peak in spring and early summer. Feathers are also used in communication. For example, when a cockatoo raises its crest it is expressing surprise or fright. In another example, a male peafowl (peacock) will fan its beautiful tail to attract the female (peahen).
In summary, the functions of the feathers are: temperature regulation, protection, sexual attraction, flight, and waterproofing. Waterproofing function is possible because of a preen gland at the base of the tail that secretes oil which the bird rubs across its feathers with its beak.
Makeup of Birds
Most bird bones are pneumatic meaning they have a pocket of air in the center of the bone where the marrow would be found in a dog. This makes bird bones much lighter than mammalian bones thus allowing for birds to be light enough to fly.
The sternum or breastbone is shaped like the bottom of a boat with a keel that extends outward. The strong muscles of the pectoral region attach to the keel. These muscles power the wings during flight.
There is not a diaphragm to separate the thoracic from the abdominal cavity, therefore, birds must raise and lower their breastbone to breathe. For this reason, it is very important not to restrict the movement of the keel when restraining a bird.
The respiratory system is complex in birds. There is no diaphragm so the movement of air is different from other animals. Birds have air sacs and lungs. The movement of air and the air sacs is very complicated and efficient. Birds are very good at moving oxygen from air into the blood. Unfortunately, this also allows for the efficient absorption of toxins from the air. It is for this reason that fumes such as smoke or Teflon gas in the air easily kill birds when humans and other pets seem normal.
There are no teeth in birds. They do have saliva that hydrates the crop contents. The upper, hard palate, forms a cleft or choana, which is the communication of the nose to the mouth. The crop is a sack like organ at the base of the throat. It is the storage site for ingested food. In baby birds, it can be easily seen distended with food after the bird has been fed. It is also from this sac that parent birds regurgitate food material to feed their young. Certain cells inside the crop form a nutritious crop milk which adults, male and female, regurgitate to feed their young. This is very well developed in pigeons.
Undigested food in the crop passes through two stomachs prior to reaching the intestine. The true glandular stomach is called the proventriculus where hydrochloric acid and digestive enzymes are secreted. Food then moves to the ventriculus, or gizzard, which is a heavily muscled organ that pulverizes or grinds the food. After it is pulverized, it goes into the small intestine where nutrients are absorbed. After passing through the large intestine, colon and rectum, the feces enter the coprodeum.
Birds have a common area into which their digestive tract, urinary tract and reproductive tract empty called the cloaca. The rectum connects directly to the cloaca by the coprodeum, “copro” meaning feces, “deum” meaning space, i.e. space for waste. The dorsal compartment is the urodeum, “uro” meaning urine, “deum” meaning space, i.e. space for urine. The urodenum collects material that empty from the kidneys into the cloaca. The vent is the actual opening of the cloaca to the outside.
The kidneys excrete byproducts of metabolism, just as in humans. In birds the byproduct is uric acid, a solid substance. That is the white solid substance is visible in the droppings. There is also a clear liquid component of the urine.
The blood supply to the legs and intestines is common to the blood supply with the kidneys. Therefore, infections in the feet or intestinal tract can cause kidney disease.
If a bird has a sore foot, it needs immediate attention because the blood supply goes directly to the kidney and can cause infection to rapidly spread there. Birds can also channel blood from the legs directly to the kidneys and bypass the rest of the circulation. For this reason, injections should not be given in the legs. This could result in direct damage to the kidneys.
The reproductive system of birds is of interest. Birds have only the left ovary and oviduct that fully develop. In the female, the right ovary and oviduct atrophies. In the male, both right and left sides develop into testicles. Birds can lay eggs without ever being exposed to a male bird. These eggs are unfertilized, therefore, they won’t hatch.
Mating occurs when the male mounts the female. Their cloaca join by turning inside out. The vas deferens of the male runs from the internal testicles that are located close to the kidneys. The vas deferens empty into the dorsal part of the cloaca. There is not actually a penis in the male bird, but specialized phallic tissue that deposit semen into the female’s cloacal urodenum, where the oviduct opening is found. The semen makes its way up the oviduct to fertilize the eggs.
In birds, the nerves that supply the legs travel from the spinal cord through the kidney tissue to the leg. Any disease of the testicle, ovary or kidney can present as lameness. This is a common presentation for kidney tumors in budgies.
Although some birds don’t have a gall bladder, most of the other internal organs are similar in appearance and function as that same organ in a dog or cat.
Birds do have ears, but the ears do not have external flaps or pinna like in a dog.
Some birds have specialized sensory feathers around the ears that may be confused with ears. For example, even Hoot Owls have feathers that look like ears but they are feathers designed to channel sound into the ear canal. That is why owls can hear so well that they can hear a mouse rustling a mile away!
Birds have nucleated red blood cells. Red blood cells with a blue nucleus in the center is normal. There are other differences in the white blood cells. In particular, the cell we refer to as a neutrophil in dogs is referred to as a heterophil in birds. These cells look a little different but have the same function.