Muscles and organs

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.

The origin of feathers

Feathers are complex and novel evolutionary structures. They did not evolve directly from reptilian scales, as once was thought. Current hypotheses propose that they evolved through an invagination of the epidermis around the base of a dermal papilla, followed by increasing complexity of form and function. They evolved before birds and even before avian flight. Thus, early feathers functioned in thermal insulation, communication, or water repellency, but not in aerodynamics and flight. Among extinct life-forms, feathers are no longer considered a unique and diagnostic characteristic of birds. Feathers with modern features were present in a variety of forms on a variety of theropod dinosaurs. At least nine Cretaceous dinosaurs had featherlike structures. The details of some are questionable, but some, such as those of Sinornithosaurus and other basal dromaeosaurs, bear a resemblance to modern pennaceous feathers. Feathered dinosaurs did not survive the end of the Cretaceous Period, but birds did, and then they flourished.

The origin of flight

Experts continue to debate whether flight evolved through gliding by an arboreal ancestral bird or through aerial launching by a running terrestrial ancestor. Historically these two hypotheses have been strongly linked to, respectively, the thecodont origin hypothesis and theropod origin hypothesis. The shift of opinion toward the theropod hypothesis, however, does not resolve this debate, since feathers on the forelimbs of early birds could have facilitated the early stages of flight through either mode. Precursors of an effective flight stroke of the forelimbs were present in terrestrial bipedal theropods. In either case, the evolution of avian flight required a decoupling of coordinated movements of the forelimbs and hind limbs. It also depended on new neural links between forelimb and tail movements as well as on other elaborations essential to controlled flight without major (initial) compromises of terrestrial locomotion. Once controlled flight had evolved, the avian body plan was transformed into a powerful flight engine. The transformation was then followed by the loss of other capabilities—or, in some cases, of flight itself.

Fossil birds

Archaeopteryx lithographica, which was first discovered in southern Germany in 1861, was once known as the world’s earliest fossil bird. A series of fossils, each dated to approximately 150 million years ago during the Jurassic Period, were about the size of a magpie. It resembled some reptiles and differed from modern birds in many ways, notably: (1) the jaw contained teeth set in sockets, (2) the sternum was short and not keeled, (3) the bones were not pneumatic, (4) the first two metacarpals were free, resulting in three movable digits of the “hand,” all with functional claws. Avian characteristics of Archaeopteryx included the possession of feathers and other skeletal features indicating that the creature represented an intermediate stage between reptiles and modern birds. The absence of a keel on the short sternum indicates that Archaeopteryx did not fly but glided. An opposable hallux, indicative of a perching type of foot, and clawed digits on the hand point to an arboreal existence.

However, early in the 21st century, the discovery of Xiaotingia zhengi, a fossilized feathered dinosaur in China that shares several birdlike characteristics with Archaeopteryx, cast doubt on the notion that all birds descended from Archaeopteryx. X. zhengi appears in the fossil record some five million years before Archaeopteryx, and this fact suggests that the single ancestor from which all bird lineages originate has yet to be found. The earliest fossil bird known to be ancestral to the lineage of modern birds is Archaeornithura meemannae. The species was described in 2015 after having been found in rocks of the Huajiying Formation of northeastern China that date to 130.7 million years ago.

The other major group of toothed Cretaceous birds, the Odontornithes, included one of the best-known groups of fossil birds, Hesperornis and its relatives. These birds were highly specialized foot-propelled divers of the Late Cretaceous. Hesperornis was up to 1.8 metres (6 feet) long and had completely lost the power of flight. The sternum lacked a keel, the humerus was small and weak, and other elements of the wing were missing entirely. The pelvis and hind limb had a strong but superficial resemblance to those of modern loons and grebes. However, two major features and several less obvious ones indicate that the resemblance was the result of convergent evolution rather than common ancestry. Hesperornis was remarkable for three features. It had (1) teeth set in grooves, not sockets, (2) a stout fourth toe with a unique rotary ball-and-flange type of articulation, and (3) tail (caudal) vertebrae with limited vertical motion, making the tail somewhat beaverlike in its action. Baptornis, a contemporary relative of Hesperornis, was smaller and less strongly modified. Though flightless, its wings were less reduced than Hesperornis, and it lacked the peculiar modifications of the fourth toe and caudal vertebrae.

In the Late Cretaceous also appeared the first modern birds, assigned to the infraclass Neornithes, or Carinata. Living alongside Hesperornis and other Odontornithes was a group of flying birds that included Ichthyornis and Apatornis. Although not related to gulls, these birds resembled them superficially and may well have been their ecological counterparts. It was long believed that Ichthyornis had teeth, like Hesperornis, but it is now thought that the toothed jaws formerly thought to belong to Ichthyornis were really those of a small mosasaur, a marine reptile.

A genetic study of bird lineages estimated that birds began to lose the outer covering of enamel from their teeth, and possibly their teeth as well, some 116 million years ago during the Early Cretaceous. The study also showed that this change was accompanied by an increase in beak development that assisted in the diversification of living birds.

Modern birds

In the evolution of modern birds from an Archaeopteryx-like form, the development of active flight must have occurred early. This meant an increase in size of the muscles moving the wing and the development of a keel on the sternum as an added area of attachment for these muscles. As the tail took on more of a steering function and less of a supportive one, it became shorter and more readily moved as a unit. Feathers became specialized for different functions, and at the same time the eyes, brain, and respiratory and circulatory systems continued to develop in a manner associated with the evolution of homeothermic, arboreal, gliding animals. By the time birds became strong fliers, they were ready to exploit many new environments, and by the Cretaceous Period they had begun to do so, producing the wide array of adaptive types known today.

The major diversification of modern birds probably took place in the Cretaceous, and it must have started early in that period because fragmentary fossil evidence of foot-propelled divers (Enaliornis) and of an early relative of the flamingos (Gallornis) are known from Lower Cretaceous deposits in Europe. Upper Cretaceous deposits have yielded, besides Hesperornis and Ichthyornis and their relatives, diving birds similar to Enaliornis, other early flamingo-like birds, and species in the same suborders as gannets, ibises, rails, and shorebirds.

Deposits from the Paleocene Epoch (65.5 million to 55.8 million years ago) have yielded the earliest known loons, cormorants, New World vultures, and gulls. In addition, large, flightless predatory birds culminating in Diatryma made their appearance during this period. From the far richer Eocene Epoch (55.8 million to 33.9 million years ago) have come the earliest known fossil representatives of most of today’s bird orders.

Almost certainly all living orders and most living families of birds were in existence by the end of the Eocene. One of the most interesting finds from this period was fossils of Neocathartes, a long-legged bird allied to the New World vultures. There are several anatomical similarities between this group of vultures and the storks, and the existence of this fossil lends support to the idea that the storks and New World vultures are more closely related to each other than each family is to the birds with which it is usually grouped.

By Pliocene times (5.3 million to 2.6 million years ago), most modern genera were probably in existence. After the extinction of the dinosaurs and before large carnivorous mammals evolved, two groups of large flightless birds evolved to fill a similar niche. From the late Paleocene to the middle Eocene, Diatryma and its relatives were major predators in the Northern Hemisphere. The largest species stood over 2.25 metres (7 feet) tall and had stout hooked beaks. They are of uncertain relationships but may have been distantly related to the cranes and rails (order Gruiformes). The second group, that of Phororhacos and related genera, had a long history (from the lower Oligocene to the middle Pliocene) in South America, which was without large carnivores until relatively recent times. The Phororhacos line evidently evolved from seriema-like stock and radiated into numerous genera and species, the largest of them (Onactornis) standing 2.5 metres (8 feet) tall and having a skull 80 cm (31 inches) long and 40 cm (16 inches) high.

Large grazing or browsing birds appear to have evolved several times. On continents where there are large predators, these birds have always been rapid runners (ostriches, rheas, emus), but on islands lacking such predators, they were slow-moving, heavy-bodied birds. Two such groups were the elephant birds of Madagascar and the moas of New Zealand, the largest in each group approaching 3 metres (10 feet) in height. Fragmentary fossil material from Eocene and Oligocene deposits in Egypt indicates that similarly adapted birds occurred there before the advent of large carnivores.

Distinguishing taxonomic features

In classifying birds, most systematists have historically relied upon structural characteristics to infer evolutionary relationships. Plumage characteristics include the number of various feather types; the presence or absence of down on the feather tracts and on the preen gland; and the presence or absence of an aftershaft. Characteristics of the bill and feet are also useful, as is the arrangement of bones in the palate and around the nostrils. The presence or absence of certain thigh muscles is considered, as are the arrangement of the carotid arteries, the syrinx, and the deep flexor tendons of the toes as well as the condition of the young when hatched. Advances in the study of DNA sequences and computerized construction of phylogenetic trees have provided new means of testing hypotheses of taxonomic relationships.

Critical appraisal

It has frequently been stated that birds are one of the best known of animal groups. This is true in the sense that most of the living species and subspecies in the world have probably been described; but because of inadequacies in the fossil record and repeated cases of convergent evolution within the group, our knowledge of the phylogenetic relationships between orders, suborders, and families of birds is inferior to that of mammals and reptiles. Most, if not all, of the major lineages of modern birds arose rapidly in the Late Cretaceous and the Paleogene Period (about 100 million to 23 million years ago). DNA data continue to resolve the relationships among major groups of birds. The penguins (Sphenisciformes), tube-nosed seabirds (Procellariiformes), and pelicans (Pelecaniformes) form a triad of related lineages. Waterfowl (Anseriformes) and chickenlike birds (Galliformes) are linked and together may be the oldest assemblage of modern birds. Some caprimulgiforms (owlet frogmouths) seem clearly related to swifts (Apodiformes) through a link between owlet frogmouths and treeswifts.

The taxonomic positions of several bird groups remain open to question. The hoatzin, included below in the Cuculiformes, is often given its own order, Opisthocomiformes. The sandgrouse are listed separately in order Pteroclidiformes. The turacos, sometimes included in the Cuculiformes, are considered by many authors to warrant separation and are listed here as Musophagiformes. Diatryma and several related genera of extinct flightless predators are often placed in a distinct order, Diatrymiformes, near Gruiformes. The flamingos, which constitute the order Phoenicopteriformes in some classifications, are placed in the Ciconiiformes in this classification, but their relationships are still unknown.

One area particularly in need of study is the relationships among the various groups of ratites (ostriches, rheas, emus, moas, and others). Formerly, some authorities argued that these birds and the penguins arose independently from cursorial reptiles, but it is now generally agreed that all of them passed through a flying stage in the course of their evolution. The ratite groups differ greatly in morphology and yet show remarkable similarities in palate and bill characters. The principal unanswered questions are how many different flightless lines evolved from flying ancestors and from how many different groups the flying ancestors evolved. On zoogeographic grounds, it is likely that the isolated kiwi-moa, elephant bird, and emu-cassowary lines arose independently from each other and from ratites on the other continents. But the ostriches and rheas could be descended from a common flightless ancestor because of the known former land connections from Asia to North and South America. Kiwis, ostriches, rheas, emus, and cassowaries are contained within order Struthioniformes in this classification.

The evolutionary sequence of the bird orders starts with ratites and marine seabirds and ends with songbirds. Beginning in the 1980s, Charles Sibley proposed radically different listings of the nonpasserine orders on the basis of his pioneering DNA analyses.

Annotated classification

This classification is a synthesis of current information compiled by American ornithologist Frank Gill (2002).

• Class Aves (birds)

  10,100 living species of vertebrate (backboned) animals primarily adapted for flight with feathers. Warm-blooded with a 4-chambered heart; left systemic arch lost. Lower jaw articulates with cranium via the quadrate; teeth absent in living forms. Reproduction by hard-shelled eggs, nearly always incubated by one or both parents.

  • Order Passeriformes (songbirds, or perching birds)

    5,700 species in 74 families (depending on the authority), worldwide; complex assemblage containing more than half of all known bird species; bill, plumage, and habits highly varied; length 7.5–125 cm (3–49 inches).

  • Order Apodiformes (swifts, hummingbirds)

    Approximately 425 species in 3 families including crested swifts, worldwide except in the extreme north; hummingbirds limited to New World; rapid-flying birds that feed in flight upon insects or nectar; “hand” and primary flight feathers constitute a relatively great proportion of the wing; feet weak; length 6.3–23 cm (2.5–9.1 inches).

  • Order Piciformes (woodpeckers and allies)

    Approximately 400 species in 6 families including jacamars, puffbirds, barbets, honey guides, toucans; worldwide in forests; hole-nesting birds that feed upon insects and fruit; outer toes able to face rearward; woodpeckers specialized for climbing; honey guides are brood parasites; length 7.5–61 cm (3–24 inches).

  • Order Charadriiformes (gulls, sandpipers, auks, and allies)

    370 species in 17 families including plovers, jacanas, stilts, avocets, thickknees, terns, and murres; worldwide. Three basic body plans: suborder Charadrii—waders (shorebirds) that usually feed on small animals in mud or water; bill variable but often long and used for probing; Lari—web-footed, dense-plumaged water birds that feed by plunging into water for fish, robbing other birds, or scavenging; Alcae—dense-plumaged, web-footed, marine, wing-propelled divers that feed on fish or invertebrates; length 12–78 cm (4.7–30.7 inches).

  • Order Pteroclidiformes (sandgrouse)

    16 species in 1 family. Stocky, pigeonlike ground birds with short legs but fast flight; feed on seeds and insects; deserts of Africa and Asia; length 22–40 cm (about 9–16 inches).

  • Order Psittaciformes (parrots, lorikeets, cockatoos, kea, and kakapo)

    About 368 species in 2 families, 10 species extinct since 1600; tropical, with some temperate-zone species; often brightly coloured; strong-flying, seed-, fruit-, or nectar-eating birds with very stout, hooked bills and zygodactyl feet (i.e., outer toe facing rearward); length 8–100 cm (3.2–39 inches).

  • Order Columbiformes (pigeons and doves)

    300-plus species in 1 family, worldwide except in the extreme north; fast-flying birds with pointed wings and weak bills; feed on seeds and fruit; length 15–120 cm (5.9–47.2 inches).

  • Order Falconiformes (diurnal birds of prey)

    309 species in 5 families including hawks, falcons, eagles, the secretary bird, Old World vultures, and condors; length 14–150 cm (5.5–59 inches), condor wingspan more than 3 metres (10 feet); some fossil forms larger.

  • Order Galliformes (chickenlike birds)

    About 290 species in 5 families including pheasants, megapodes, guinea fowl, curassows, and guans; nearly worldwide, except southern South America; terrestrial or arboreal, with strong, scratching feet; short, rounded wings; feathers with long aftershafts; length 15 to more than 200 cm (5.9 to more than 79 inches).

  • Order Gruiformes (cranes and allies)

    About 210 species in 11 families including rails, coots, moorhens; worldwide and diverse group, ranging from small quail-like hemipodes to large long-legged cranes, marsh-inhabiting rails, swimming coots and finfoots, and cursorial bustards; length 12–176 cm (4.7–70 inches). The carnivorous phororhacoids of the early Cenozoic Era belong here, as may the very large Diatryma and its relatives; fossils to 200 cm (6.6 feet) tall.

  • Order Procellariiformes (tubenosed seabirds)

    117 species in 4 families including albatrosses, shearwaters, and petrels; oceans worldwide but most numerous in Southern Hemisphere; web-footed marine birds with tubular nostrils; possess a musky smell; most have narrow wings and stiff, gliding flight; length 13–200 cm (5.1–79 inches), albatross wingspan more than 3 metres (10 feet).

  • Order Coraciiformes (kingfishers and allies)

    211 species in 10 families including hornbills, bee-eaters, rollers, hoopoes, todies, motmots; worldwide except in the extreme north; heterogeneous group of hole-nesting birds; many with long, pointed bills and blue or green in plumage; all have 2nd and 3rd or 3rd and 4th toes joined at base; food largely animal, except hornbills, which eat much fruit; length 10–120 cm (4–47 inches).

  • Order Strigiformes (owls)

    180 species in 2 families worldwide, nocturnal raptorial birds with hooked beaks, strong talons, and soft plumage; length 12–69 cm (4.7–30 inches).

  • Order Musophagiformes (turacos)

    18 species in 1 family, colourful plumage, fruit-eating; length 35–70 cm (14–28 inches); Africa.

  • Order Cuculiformes (cuckoos and allies)

    141 species in 2 families including anis, roadrunners, and the hoatzin; one species extinct since 1600; worldwide except in the extreme north; long-tailed birds with rearward or sideward facing toes; feed on both fruits and small animals; most arboreal, a few terrestrial; some are brood parasites; length 16–76 cm (6.3–30 inches).

  • Order Anseriformes (screamers, waterfowl)

    150 species 2 families worldwide, including ducks, geese, and swans; web-footed birds with broad bills containing fine plates or lamellae except for screamers, large-footed marsh birds with chickenlike bills; length 34–180 cm (13–71 inches).

  • Order Ciconiiformes (herons, storks, and allies)

    120 species in 6 families including shoebills, New World vultures, ibises, bitterns; worldwide except in the extreme north; long-legged wading birds with long bills; feet not webbed; length 25–152 cm (9.7–60 inches).

  • Order Caprimulgiformes (nightjars)

    121 species in 5 families including frogmouths, potoos, and the oilbird; worldwide except in the extreme north; nocturnal and concealingly coloured, with weak feet, soft plumage, and very large mouths; most feed on insects caught in flight; length 15–60 cm (6–24 inches).

  • Order Pelecaniformes (pelicans and allies)

    66 species in 6 families worldwide, including cormorants, boobies, gannets, tropic birds, and frigate birds. Water birds with all 4 toes webbed; bill hooked or straight and sharply pointed; length 48–188 cm (19–74 inches).

  • Order Tinamiformes (tinamous)

    47 species in 1 family; Central and South America; ground-dwelling birds resembling quails or pheasants with flat, elongated, and rather weak bills and very small tails; length 20–53 cm (8–21 inches).

  • Order Trogoniformes (trogons)

    37 species in 1 family; tropical, except Australasia; extremely soft-plumaged arboreal birds that feed on insects and small fruit; feet weak; 1st and 2nd toes directed backward; length 23–40 cm (9.1–16 inches).

  • Order Podicipediformes (grebes)

    22 species in 1 family worldwide, 2 species recently extinct; foot-propelled diving birds with lobed toes, minute tails, and silky plumage; length 20–78 cm (8–31 inches).

  • Order Sphenisciformes (penguins)

    17 species in 1 family in oceans of the Southern Hemisphere; wings flipperlike for propulsion underwater; webbed feet short and stout; stance upright; feathers short and dense, molted in patches; length 35–115 cm (14–45 inches); fossil forms to 180 cm (71 inches).

  • Order Gaviiformes (loons)

    5 species in 1 family of the Northern Hemisphere; foot-propelled diving birds with webbed feet and pointed bills; length 53–91 cm (21–36 inches).

  • Order Coliiformes (colies, or mousebirds)

    6 species in 1 family of Africa south of the Sahara; soft plumage with long, pointed tails and all 4 toes directed forward; largely vegetarian, some insects; length 29–36 cm (11–14 inches).

  • Order Struthioniformes (ostriches, rheas, emus, cassowaries, and kiwis)

    10 species in 6 families in Africa, South America, New Zealand, Australia, and Oceania, with fossils from southern Europe and Asia, including India and Mongolia; cursorial (running); height 35 cm to 2.7 metres (14 inches to almost 9 feet). Many species have small tails with little or no aftershaft. Some forms are nearly wingless. Order includes the largest living birds.