Bird anatomy

Bird anatomy shows so many unusual adaptations (mostly aiding flight) that birds have earned their own unique class in the vertebrate phylum.

Respiratory system

Due to having the high metabolic rate required for flying, birds have a high oxygen demand. They meet this by having a respiratory system more efficient than that of a mammal or a reptile. Birds ventilate their lungs by means of posterior and anterior air sacs (typically nine) which act like bellows, but do not play a direct role in gas exchange. The lungs have a fixed volume and are the site of gas exchange, the air passing through on its way to the air sacs and on its way back from the air sacs.

There are three distinct sets of organs involved in respiration:

The posterior and anterior air sacs expand during inhalation. Air enters the bird via the trachea. Half of the inhaled air enters the posterior air sacs, the other half passes through the lungs and into the anterior air sacs.

The posterior and anterior air sacs contract during exhalation. The anterior air sacs empty directly into the trachea, the posterior air sacs empty via the lungs, the lungs expel this air via the trachea.

Since during inhalation and exhalation fresh air flows through the lungs in only one direction, there is no mixing of oxygen rich air and carbon dioxide rich air within the lungs as in mammals. Thus the partial pressure of oxygen in a bird's lungs is the same as the environment, and so birds have more efficient gas-exchange of both oxygen and carbon dioxide than do mammals.

Avian lungs do not have alveoli, as mammalian lungs do, but instead contain millions of tiny passages known as parabronchi, connected at either ends by the dorsobronchi and ventrobronchi. Air flows through the honeycombed walls of the parabronchi and into air capillaries, where oxygen and carbon dioxide are traded with cross-flowing blood capillaries by diffusion.

A diaphragm is absent in birds; the entire body cavity acts as a bellows to move air through the lungs. The active phase of respiration in birds is exhalation, requiring effort of the musculature.

Circulatory system

Birds have four chambered hearts, in common with humans, most mammals and some reptiles. This adaptation allows for efficient nutrient dispersion and oxygen transportation, throughout the body, which provides birds with the energy they need to fly and to lead highly active lives. A hummingbird's heart beats at a rate of 1000 beats per minute (about 16.5 beats per second).

Digestive system

Birds possess a ventriculus, or gizzard, that is composed of four muscular bands that act to rotate and crush food by shifting the food from one area to the next within the gizzard. Depending on the species, the gizzard may contain small pieces of grit or stone that the bird has swallowed to aid in the grinding process of digestion. For birds in captivity, only certain species of birds require grit in their diet for digestion. The use of gizzard stones is a similarity between birds and dinosaurs, which left gizzard stones called gastroliths as trace fossils.

The skeleton of a dove.

Skeletal system

Muscular system

There are about 175 different muscles in the bird. They mainly control the wings, the skin and the legs, but also many other parts of the bird. The largest muscles in the bird are the muscles that control the wings. They are called the pectorals, or the breast muscles, and make up about 15 - 25% of a bird’s full body weight. They make the birds’ wing stroke very powerful so that they can fly, and provide most of the movements the bird needs for its down stroke. The muscle below the pectorals is the supracoracoideus. It raises the wing when a bird is flying. The supracoracoideus and the pectorals together make up about 25 – 35% of the birds’ full body weight.

The skin muscles help a bird in its flight by making the feathers, which are attached to the skin muscle, go up, down, or move sideways. This helps the bird in its flight maneuvers.

There are only a few muscles in the trunk and the tail, but they are very strong and are essential for the bird. The pygostyle controls all the movement in the tail and controls the feathers in the tail. This gives the tail a larger surface area which helps keep the bird in the air.

Head

Birds have acute eyesight, with raptors having vision eight times sharper than humans. This is because of many photoreceptors in the retina (up to 1,000,000 per square mm in Buteos, against 200,000 for humans), a very high number of nerves connecting the receptors to the brain, a second set of eye muscles not found in other animals, and, in birds of prey, an indented fovea which magnifies the central part of the visual field. Many species, including hummingbirds and albatrosses, have two foveas in each eye, and the ability to detect polarised light is also common.

Birds have a large brain to body mass ratio. This is reflected in the surprisingly advanced and complex bird intelligence.

The region between the eye and bill on the side of a bird's head is called the lores. This region is sometimes featherless, and the skin may be tinted (as in many species of the cormorant family).

Reproduction

Although most male birds have no external sex organs, the male does have two testes which become hundreds of times larger during the breeding season to produce sperm. The female's ovaries also become larger, although only the left ovary actually functions.

In the males of species without a phallus (see below), sperm is stored in the seminal glomera within the cloacal protuberance prior to copulation. During copulation, the female moves her tail to the side and the male either mounts the female from behind or in front (in the stitchbird), or moves very close to her. The cloacae then touch, so that the sperm can enter the female's reproductive tract. This can happen very fast, sometimes in less than one second.

The sperm is stored in the female's sperm storage tubules for anywhere from a week to a year, depending on the species of bird. Then, one by one, eggs will be fertilized as they come out of the ovaries, before being laid by the female. The eggs will then continue their development outside the female body.

Many waterfowl and some other birds, such as the ostrich and turkey, do possess a phallus. Except during copulation, it is hidden within the proctodeum compartment within the cloaca, just inside the vent.

After the eggs hatch, parent birds provide varying degrees of care in terms of food and protection.

 Precocial birds can care for themselves independently within minutes of hatching. Altricial hatchlings are helpless, blind, and naked, and require extended parental care.

Precocial birds

In biology, precocial species are those that are relatively mature and mobile from the moment of birth or hatching. Precocial species are normally nidifugous, meaning that they leave the nest shortly after birth.

The distinction between precocial and altricial species is particularly sharp in birds. Precocial birds, including many ground-nesting species, have offspring that are born covered with down and ready to leave the nest in a short period of time following hatching (e.g. 24 hours). This amount of time is necessitated by the fact that offspring require time for their downy coats to dry prior to leaving the nest. Additionally, most precocial chicks lack thermoregulation (the ability to regulate their own body temperatures), and they depend on the attending parent(s) to provide an outside source of heat for a short period of time. Precocial birds find their own food. Examples of precocial birds include the domestic chicken, many species of ducks and geese, and rails and crakes.

Precocial species typically have a longer gestation or incubation period than related altricial species, and smaller litters or clutches, since each offspring has to be brought to a relatively advanced (and large) state before birth or hatching.
 

Altricial hatchlings

Altricial hatchlings are helpless, blind, and naked, and require extended parental care. The chicks of many ground-nesting birds such as partridges and waders are often able to run virtually immediately after hatching; such birds are referred to as nidifugous. The young of hole-nesters, on the other hand, are often totally incapable of unassisted survival. The process whereby a chick acquires feathers until it can fly is called "fledging".

Some birds, such as pigeons, geese, and Red-crowned Cranes, remain with their mates for life (or for a long period) and may produce offspring on a regular basis.

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