The cardiac (heart) muscles and smooth muscles of the viscera of birds resemble those of reptiles and mammals. The smooth muscles in the skin include a series of minute feather muscles, usually a pair running from a feather follicle to each of the four surrounding follicles. Some of these muscles act to raise the feathers, others to depress them. The striated (striped) muscles that move the limbs are concentrated on the girdles and the proximal parts of the limbs. Two pairs of large muscles move the wings in flight: the pectoralis, which lowers the wing, and the supracoracoideus, which raises it. The latter lies in the angle between the keel and the plate of the sternum and along the coracoid. It achieves a pulleylike action by means of a tendon that passes through the canal at the junction of the coracoid, furcula, and scapula and attaches to the dorsal side of the head of the humerus. The pectoralis lies over the supracoracoideus and attaches directly to the head of the humerus. In most birds the supracoracoideus is much smaller than the pectoralis, weighing as little as one-twentieth as much; in the few groups that use a powered upstroke of the wings (penguins, auks, swifts, hummingbirds, and a few others), the supracoracoideus is relatively large.

Avian striated muscles contain a respiratory pigment, myoglobin. There are relatively few myoglobin-containing cells in “white meat,” whereas “dark meat” derives its characteristic colour from their presence. The former type of muscle is used in short, rapid bursts of activity, whereas the latter is characteristic of muscles used continuously for long periods and especially in muscles used during diving.

The circulatory system of birds is advanced over that of reptiles in several ways: (1) there is a complete separation between pulmonary circulation (lungs) and systemic (body) circulation, as in the mammals, (2) the left systemic arch (aortic artery) is lost, blood passing from the heart to the dorsal aorta via the right arch, (3) the postcaval vein is directly connected with the renal portal that connects the kidneys with the liver, and (4) the portal circulation through the kidneys is greatly reduced. Birds’ hearts are large—0.2 to over 2.4 percent of body weight, as opposed to 0.24 to 0.79 percent in most mammals.

The avian lung differs from the type found in other land vertebrates in that several pairs of nonvascular air sacs are connected with the lungs. These extend into the pneumatic parts of the skeleton. Muscles between the sternum and trachea or along the trachea and bronchi vary tension on the membranes.

The avian digestive system shows adaptations for a high metabolic rate and flight. Enlargements of the esophagus, collectively called the crop, permit the temporary storage of food prior to digestion. The stomach is typically divided into a glandular proventriculus and a muscular gizzard, the latter lying near the centre of gravity of the bird and compensating for the lack of teeth and the generally weak jaw musculature. Otherwise, the digestive system does not vary markedly from the general vertebrate type.

Like reptiles, birds possess a cloaca, a chamber that receives digestive and metabolic wastes and reproductive products. A dorsal outpocketing of the cloaca, the bursa of Fabricius, controls antibody-mediated immunity in young birds. The bursa regresses with age, and thus its presence or absence may be used to determine age.

The testes of the male bird are internal, like those of reptiles. Intromittent organs are found in only a few groups (waterfowl, curassows, tinamous, ratites). The distal part of the vas deferens (the seminal sac) becomes enlarged and convoluted in the breeding season and takes on both secretory and storage functions. In songbirds this enlargement and the adjacent part of the cloaca form a cloacal protuberance, a swelling visible on the outside of the bird. Usually only the left ovary and oviduct are functional. As the egg moves down the oviduct, the albumen, membranes, and shell are laid down. The gonads and accessory sexual organs of both sexes enlarge and regress seasonally. In the breeding season, the testes of finches may increase more than 300 times their winter size.

Birds are homeothermic (warm-blooded) and maintain a body temperature of approximately 41 °C (106 °F). This temperature may be slightly less during periods of sleep and slightly higher during intense activity. Feathers, including down, provide effective insulation. In addition, layers of subcutaneous fat add further insulation in penguins and some other water birds. Heat loss through the feet in cold weather is minimized by reducing blood flow to the feet and by a heat-exchange network in the blood vessels of the upper leg, so that the temperature of blood flowing into the unfeathered part of the leg is very low.

Birds do not possess sweat glands. Excess heat is dissipated by rapid panting, which reaches 300 respirations per minute in domestic hens. Some heat can also be lost by regulation of blood flow to the feet. In hot climates, overheating is often prevented or reduced via behavioral means by concentrating activities in the cooler parts of the day and seeking shade during the hot periods. Temporary hypothermia (lowered body temperature) and torpor are known for several species of nightjars, swifts, and hummingbirds. Torpor at night is believed to be widespread among hummingbirds. The heart rate of birds varies widely—from 60 to 70 beats per minute in the ostrich to more than 1,000 in some hummingbirds.

The kidneys lie in depressions that are located on the underside of the pelvis. The malpighian bodies, which are the active tubules of the kidney, are very small in comparison with those of mammals, ranging from 90 to 400 per cubic mm. More than 60 percent of the waste nitrogen is excreted as uric acid or its salts. There is some reabsorption of water from the urine in the cloaca, with uric acid remaining. There is no urinary bladder, the urine being voided with the feces. In marine birds, salt is excreted in a solution from glands lying above the eyes through ducts leading to the nasal cavity.

The origin of birds

The debate over the origin of birds centres on whether birds descended directly from thecodont reptiles about 230 million years ago (during the Triassic Period) or from a later lineage, the carnivorous theropod dinosaurs. This debate has been long-standing and divisive. At the beginning of the 21st century, the pendulum has swung decisively toward the theropod ancestor hypothesis—that today’s birds are feathered dinosaurs. This hypothesis is supported by analyses of shared characteristics (synapomorphies) combined with improved samples of early bipedal theropods.