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 anterior air sacs (interclavicular, cervicals, and anterior thoracics), the lungs, and the posterior air sacs (posterior thoracics and abdominals).
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 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.
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 Ruby-throated Hummingbird's heart beats up to a rate of 1200 beats per minute (about 20 beats per second).
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 bird skeleton is highly adapted to the capacity for flight. It is extremely lightweight but strong enough to withstand the stresses that a bird experiences, when taking off, flying or landing. One of the adaptations that make this possible is the fusing of bones that are separate in mammals, into single ossifications, such as the pygostyle. Because of this, birds usually have a smaller number of bones than mammals or reptiles.
Birds have a jaw that has adapted into a beak, on which baby birds have an egg tooth.
Birds have many bones that are hollow, with criss-crossing struts or trusses (cross walls) for structural strength. (Some flightless birds like penguins have only solid bones, however). The number of hollow bones varies from species to species, though large gliding and soaring birds tend to have the most. Most bones contain oxygen which also makes them lighter. Birds also have more cervical (neck) vertebrae than many other animals; most have a highly flexible neck that consists of 13-25 vertebrae. Birds are the only vertebrate animals to have a fused collarbone (the furcula or wishbone) or a keeled breastbone.
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.
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).
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 fertilised 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. When not copulating, 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. 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.
- ^ June Osborne (1998). The Ruby-Throated Hummingbird. University of Texas Press, 14. ISBN 0292760477.