Birds

Birds are a class (Aves) within the Animal Kingdom that contains almost 11,000 species. Each of these contains feathers, which distinguishes them from all other creatures on Earth. Birds also have horny beaks and two hind limbs that allow them to walk, swim, or perch. Birds also have two wings, or forelimbs, which help most species to fly or swim. The anatomical systems in birds are similar to those of mammals, both warm-blooded or endothermic. Along with other vertebrates, birds have skeletal, muscular, circulatory, digestive, respiratory, urinary, reproductive, and nervous systems. They have an outer covering of skin and a variety of sensory organs. Each system, however, has unique adaptations designed to enable birds to fly.

Avian Physiology

Feathers are a lightweight body covering that insulates and protects birds. The large, vaned feathers on the wings and tails provide most birds with the aerodynamic lift and maneuverability needed for flight. The shape, color, and size of some feathers is also important for camouflage, recognition, or behavioral displays. The skeletal systems of birds have many bones that are homologous to reptilian or mammalian bones, yet unique modifications give birds the strength, flexibility, and lightness to fly. Most birds have bones that are pneumatized or filled with air. The supporting bones, however, are highly mineralized and very strong. Some bones that are found in reptiles and mammals are missing in birds; other bones are fused. There are also more cervical vertebrae in birds’ necks, and they are unusually flexible, allowing great movement of the head and neck.

The circulatory systems of birds are very similar to those of mammals. In addition, birds have a four-chambered heart. The oxygen-carrying erythrocytes of birds are larger than those of mammals and, unlike those of mammals, have a nucleus. Heartbeat rates are a function of activity, environmental temperature, and size. A hummingbird’s heart often beats more than one thousand times per minute.

A bird's beak may serve many functions, including probing for, catching, crushing, tearing, and swallowing food. The toothless beak does not grind or chew food but passes it to the pharynx, next to the esophagus, and then to a storage chamber, the crop. Food eventually is passed to a glandular stomach, where digestion begins, and then to a muscular gizzard that, with the aid of a horny lining and grit or sand, grinds and digests it further. The gizzard leads to the intestine, where secretions from the liver and pancreas aid digestion. The feces empty into a cloacal chamber. The cloacal chamber is also the terminus of the urinary and reproductive systems. The cloaca is the only posterior opening in birds; since all sex organs are internal, including the penis and testes, the sex of a bird cannot usually be determined by examining this area.

The visual and auditory sense organs of birds are usually acutely developed. Senses of smell, taste, and touch, on the other hand, are often poorly developed. Pigeons can discriminate motions that are up to three times faster than any humans can distinguish; many birds, like hummingbirds, can see ultraviolet light, which is invisible to humans. A bird's view of a rainbow consists of a much wider arc of color than that of humans, including more shades of indigo and violet. However, diurnal birds need up to 20 times more light than humans to see properly. Eagles, hawks, and vultures can detect objects at remarkable distances; they have a retinal cell density that is twice that of humans. Oilbirds and some owls can locate prey in total darkness.

These anatomical systems and the varied external sizes, shapes, and colors, as well as behaviors that include courtship patterns, migration, and vocalization, evolved during millions of years of avian evolution. Birds are thought to have evolved from reptiles, probably thecodonts, more than 150 million years ago. Several fossils of the earliest known bird, Archaeopteryx, have been dated at 150 million years old. The fossils clearly show feathers and wings on a crow-sized bird body that has teeth and some reptilian skeletal features.

Types of Birds

Evidence of between 150,000 and 1.5 million birds have existed according to the fossil record, though only around 11,000 exist in the twenty-first century. Two extinct bird groups were especially common about 90 million years ago—the large, toothed, cormorant-like birds diving birds in the genus Hesperornis and the toothy gull-like birds in the genus Ichthyornis. Two groups of huge flightless birds have become extinct within historical times: the north and south island giant moas (Dinornis robustus and Dinornis novaezelandiae) of New Zealand (about 3 meters tall), and the elephant birds (order Aepyornithiformes) of Madagascar (about 2.7 meters tall).

Birds belong to the taxonomical class Aves. This class is divided further. The first division is into orders, which are groups of birds that have broadly similar characteristics. Orders are then divided into about 250 families, families into over 2,000 genera (singular genus), and then into species. The largest family of birds, tyrant flycatchers (Tyrannidae), consists of over 400 species.

Flightless and Perching Birds

There are several orders of large, flightless, running birds in the infraclass Palaeognathae called ratites. These include Struthioniformes, with two living species that are the largest living birds: the 2.5-meter-tall ostriches of Africa, including the Somali ostrich (Struthio molybdophanes) and the common ostrich (Struthio camelus). Somewhat similar in appearance but with differences great enough for them to be placed in a different order are the two species of rheas, belonging to the order Rheiformes, including the near-threatened greater rhea (Rhea americana) and Darwin's rhea (Rhea pennata). Several of these South American birds stand 1.5 meters tall. Australia has two superficially similar groups of birds in the order Casuariiformes: the three species of tropical forest-dwelling cassowaries, which are sometimes more than 1.5 meters tall, and the single species of the slightly taller emu (Dromaius novaehollandiae), which roams the open plains. The cassowaries include the southern or double-wattled cassowary (Casuarius casuarius), Northern or single-wattled cassowary (Casuarius unappendiculatus), and Dwarf or Bennett's cassowary (Casuarius bennetti). The Pygmy cassowary (Casuarius lydekkeri) is extinct.

Kiwis are native to New Zealand. Their order, Apterygiformes, contains five species of the smallest of the primitive flightless birds. Some are about thirty centimeters tall. They have no visible wings but only vestigial flaps. Their feathers look somewhat like hair, and they produce eggs that are one-quarter of their body weight—proportionally the largest egg of any existing bird. The smallest flightless bird, the Inaccessible Island rail (Laterallus rogersi), belongs to the order Gruiformes.

The largest order of entirely flightless birds, with fifteen living species, is Sphenisciformes, the penguins. This order includes the emperor penguin of Antarctica, at 1.2 meters tall, and, along the equator, the most northern penguins, the Galápagos penguin, at a little less than 60 centimeters tall.

Many other groups of birds are considered flightless or weak fliers. Around forty-five species of tinamou, of the order Tinamiformes, are weak fliers. They are thought to be most like the ancestor of all living birds. Tinamous superficially look like quail, and they are found only in Central and South America.

The largest order of birds, with 6,500 species—60 percent of all living bird species—is Passeriformes. These birds are collectively called perching birds because of their grasping, unwebbed toes—three in front and one behind—all at the same level. Most Passeriformes have anatomical features that allow them to sing very well, and they are commonly called songbirds. This diverse order includes finches, cardinals, grosbeaks, buntings, sparrows, orioles, blackbirds, tanagers, vireos, waxwings, thrushes, kinglets, mimids, warblers, wrens, shrikes, nuthatches, creepers, titmice, chickadees, swallows, crows, jays, larks, flycatchers, and starlings. Lesser-known Passeriformes include broadbills, ovenbirds, woodcreepers, antbirds, tapaculos, manakins, pittas, birds of paradise, bowerbirds, bulbuls, babblers, honey eaters, white-eyes, sunbirds, honeycreepers, waxbills, cotingas, wagtails, and flower-peckers.

Aquatic Birds and Fowl

Two bird orders are most often associated with the open oceans or the marine coastlines of continents or islands. Procellariiformes, with four families and nearly 150 living species, contains albatrosses, shearwaters, and petrels. They all have webbed feet, long, narrow wings, and nostrils consisting of raised tubes. The fifty-two species of the order Pelecaniformes are the only birds with all four toes connected by webs; most also have an expandable throat patch. This order includes pelicans, tropic birds, boobies, gannets, cormorants, frigate birds, and anhingas.

Many types of birds generally inhabit areas that border bodies of water. The bays, estuaries, salt marshes, swamps, freshwater lakes, and rivers of the world teem with a multitude of birds. Among the largest of these orders, with over 390 species, is Charadriiformes. This order contains three fairly common groups of birds. The shorebirds or waders, often seen probing their long bills into sand or mud for food, include sandpipers, lapwings, plovers, avocets, stilts, phalaropes, oystercatchers, and jacanas. The gull-like group includes the web-footed gulls, terns, jaegers, skuas, and skimmers. Several of the fifty-four species of gulls can be very cosmopolitan and are one of the most familiar sights at seaports and coastal resorts. The third group is the web-footed alcids—auks, murres, and puffins. They are northern oceanic birds that come ashore (mostly to rocky shores and cliffs) only to breed.

The twenty-two species of grebes in six genres of the Podicipedidae family make up the order Podicipediformes. These weak fliers are excellent divers and use their fleshy, lobed feet to dive for invertebrates or fish. The shallower waters of marshes and the edges of rivers and lakes are likely to have long-legged waders that belong to the order Ciconiiformes. These species of herons, egrets, storks, ibises, bitterns, and spoonbills are usually fairly large birds with long necks and bills. They probe for mollusks, crustaceans, and fish, and sometimes for reptiles, amphibians, mammals, or birds.

The rails, gallinules, crakes, coots, and cranes are the aquatic groups of over 200 species in the order Gruiformes. Standing 1.5 meters tall, some of the cranes are among the tallest of the flying birds. The heaviest of all flying birds, the kori bustard, is also in this order. It weighs about twenty kilograms (approximately fifty pounds). The bustards, button quails, and several smaller families are terrestrial Gruiformes more often found in open plains or brushlands.

The two smallest aquatic orders are Gaviiformes, the loons or divers, and Phoenicopteriformes, the flamingos. The loons are excellent divers and breed only in or along cool northern waters. Flamingos, on the other hand, are found primarily in shallow, warmer, tropical, and temperate waters. With long legs and necks, these tall waders filter small aquatic organisms through their heavy, curved bill.

Anseriformes is an order of approximately 180 species of semiaquatic birds hunted by people worldwide. Several species have been successfully domesticated. This order includes a small family of stoutly built South American birds, the screamers, which resemble pheasants with wing spurs. The larger family of waterfowl includes ducks, swans, and geese, which are found worldwide except for Antarctica. They all have webbed feet and swim well; some are also excellent divers. Another order that has been hunted and domesticated for centuries is Columbiformes. The gentle doves of this order have long been symbols of peace and, along with pigeons and sandgrouse, give the approximately 350 species of this order worldwide distribution, except for polar regions. The pet bird trade has been especially interested in several of the 410 species of Psittaciformes. The birds of this order are primarily tropical or subtropical and include parrots, cockatoos, lories, parakeets, and macaws.

The bird order that has been the most exploited, however, both by hunting and domestication for food and eggs, is Galliformes, the fowl-like birds. The almost 300 species of this order include pheasant, grouse, quail, chickens, turkeys, guinea fowls, ptarmigans, megapodes, partridges, curassows, guans, chachalacas, and the hoatzin.

Predator Birds and Other Orders

Man is not the only efficient predator of birds. Many of the 290 species of the order Falconiformes are skilled at grasping a duck off a pond, a pigeon from the air, or a sparrow from a bird feeder. The order Falconiformes includes hawks, eagles, falcons, kites, kestrels, caracaras, vultures, condors, buzzards, and the osprey and secretary bird. All have strong, sharp claws and bills and are powerful fliers.

Another order of powerful predators of vertebrates is Strigiformes, the owls. The approximately 225 species of owls are largely nocturnal, have large eyes with binocular vision, hear exceptionally well, and fly silently with unusually soft feathers. They do not tear mammals apart as the Falconiformes do but usually swallow their prey whole and later regurgitate a pellet of hair and bones.

Some 1,200 more species are included within another seven orders of birds. The order Cuculiformes contains 170 species, including cuckoos, anis, roadrunners, and coucals. Most taxonomists also place the brightly colored touracos of Africa in this order. Most of the 120 species of the order Caprimulgiformes are either nocturnal or primarily active at twilight. This order includes nighthawks, nightjars, whippoorwills, frogmouths, potoos, and the oilbird.

Two small orders of medium-sized birds with long tails are Coliiformes (mousebirds) and Trogoniformes (trogons). While the six species of mousebirds are found only in Africa, the forty-six species of trogons are distributed throughout tropical Africa, America, and Asia. Although the 300 species of the order Coraciiformes range from small to large in size, most are brightly colored and have a conspicuous bill. The kingfishers, todies, motmots, bee-eaters, rollers, hoopoes, and hornbills make up this order.

The order Apodiformes contains three superficially different groups of birds: the swifts, the treeswifts, and the hummingbirds. The 113 species of swifts are found in most areas of the world; the more than 360 species of hummingbirds live only in temperate and tropical America. Another large but more diverse order is Piciformes. These 450 species include woodpeckers, piculets, jacamars, puffbirds, barbets, honeyguides, and toucans. Some woodpeckers are cosmopolitan and regularly frequent bird feeders.

Tracking and Studying Birds in the Field

Ornithologists, the scientists who study birds, have been assisted by millions of people worldwide in their studies of bird populations and their movements. One of the largest bird projects is the Christmas bird count, sponsored by the National Audubon Society. This North American bird study has been done yearly since 1900 and involves more than forty thousand people each year. Field observers are given one day each winter to count all the birds they can find in a specified area. The information compiled about the number of birds and species distributions is published in American Birds.

More than a million birds in North America are captured each year, fitted with a small aluminum band that has a unique number and released unharmed. Some are recaptured later, sometimes years later, and perhaps hundreds or thousands of miles from the place they were banded. This technique gives information about the sizes of bird populations, mortality rates, longevity, migration routes and times, sex ratios, and age distributions. Some banders study avian parasites and sequences of feather molting. Hunters and the general public also contribute valuable information when they report dead banded birds.

Sophisticated electronic equipment has been used in many avian studies. Radio transmitters have been attached to some of the larger birds, and their individual movements have been recorded. Radar has been used to monitor both the movements of individual birds and the progress of flocks. Bird songs have been recorded and converted into graphic sonograms, which can show song differences between individuals of the same species.

Many museums have large collections of preserved or stuffed birds and bird skeletons, nests, and eggs. These may be used for anatomical or biochemical studies of birds. Taxonomists use these collections, along with studies of recently collected and live birds, to recognize species and arrange them into various classification groups.

Paleontologists studying the fossils of extinct birds have classified some nine hundred species of birds that are no longer alive. Fossils have given direct evidence about the relatedness of many bird groups; however, because they usually contain only information about bone structure, they provide limited data about species and must be interpreted carefully.

Studying Birds in the Lab

Along with appearance and anatomical structures, ornithologists now consult many areas of biology to assemble taxonomic groups. Fieldwork has provided enormous amounts of information about the ecology, behavior, singing, breeding habits, and biogeography (location) of birds. In the nineteenth century, Charles Darwin used the structure, function, and biogeography of the finches on the Galápagos Islands, Ecuador, to work out the species of these birds. These studies contributed significantly to the development of his theory of evolution by natural selection.

Fieldwork has also helped resolve many problems that have developed in the classification of birds. In 1910, bird books listed nineteen thousand species of birds. Several thousand new kinds have been discovered since then, and now almost 11,000 species are recognized. Many studies have shown that two groups of birds that look different and were at one time considered different species are, in fact, able to mate and produce offspring. These two groups are then reclassified as a single species. Each original group is considered a subspecies, race, or variety, depending on how similar they are. In North America, for example, the Baltimore oriole and yellow-shafted flicker are common in eastern areas, and the Bullock’s oriole and red-shafted flicker are common in the west. Each type of bird looks different and can easily be recognized, so they were initially classified as four species. As towns developed in mid-America, patterns of vegetation changed, and the orioles and flickers moved with the towns until the eastern and western populations finally met. Field studies in the twentieth century showed that the two flicker types were mating and producing offspring; the two orioles were, as well. The taxonomy was adjusted to reflect this new information, and now the two flickers are considered a single species, the northern flicker. Similarly, the two orioles were combined into one species, the northern oriole.

Avian biochemistry and genetics are also areas of intensive research. Studies of the size, shape, number, and staining patterns of chromosomes have been used to show the genetic relatedness of many bird species. Just as feathers, bones, and chromosomes have been shaped by the long evolutionary history leading up to each living bird, so have molecular structures been modified over time. Bird proteins have been analyzed and compared using a variety of techniques, including spectrophotometry, electrophoresis, antibody-antigen reactions, and amino acid sequencing. Studies of avian deoxyribonucleic acid (DNA) using electrophoresis, DNA hybridization, and recombinant DNA technology have also added new insights into the evolutionary relatedness of bird species. Because DNA controls the structure of all proteins and all physical structures in a bird (or in any animal), many researchers think the comparison of DNA structure provides the best overall view of species relatedness.

Popular Ornithology

There has likely been no other area of scientific inquiry that has attracted the interest of the public as the study of birds. There has been no other area of science where the public has made such significant contributions. From the forty thousand Christmas bird count participants to the two thousand bird banders in North America, millions of interested nonscientists around the world have collected a wealth of information about birds.

Birds are often described as environmental indicators, and their population numbers and health are monitored to get an idea of overall environmental integrity. The decline in the number of migrant warblers in North America, for example, is described by some ornithologists as an indication of environmental changes in their wintering grounds in tropical America, whereas other ornithologists believe that it reflects the deterioration of their breeding grounds in North America.

Birds may also be undergoing a radical change brought on by changing climatic conditions. According to a 2023 study led by the University of Michigan, birds are adapting to warmer Earth temperatures by reducing in size. Species of birds that are typically smaller are decreasing in size at a faster rate than larger birds. Explanations for the more expedient rate of change in smaller birds are still being developed, but it appears they are evolving faster. This may be consistent with a school of thought that the life spans of smaller animals are also shorter compared to larger animals. This would imply that smaller animals cycle through more generations, and hence, genetic modifications can be passed along more quickly. This can also suggest that larger species of birds are more at risk of extinction.

Conservation projects worldwide have used bird studies as a means of determining when it is necessary to fight for the protection of a particular species, subspecies, or race of birds. Major progress has been accomplished with ospreys, peregrine falcons, bald eagles, brown pelicans, trumpeter swans, whooping cranes, Kirtland’s warblers, and other birds. Yet, each project is time-consuming and costly and must be carefully carried out. The governmental designations “endangered species” and “threatened species” cover only very narrow, clearly defined taxonomic groups.

Birds have fascinated people—scientifically, aesthetically, and emotionally—perhaps more than any other group of animals. The vast data collected and the complex taxonomical classification of birds are things that can be passed on to generations to come. It can be hoped that an environment where these creatures can prosper will be passed on as well.

Principal Terms

Archaeopteryx: the earliest known bird, known only from the fossil record; it lived during the Jurassic period

Aves: the class within the phylum Chordata to which all birds, and only birds, belong

Crop: a specialized part of a bird’s digestive system that holds and softens food

Endotherm: an animal that, by its own metabolism, maintains a constant body temperature (is “warm-blooded”); birds and mammals are endotherms

Gizzard: a part of a bird’s stomach that uses ingested pebbles to grind up food

Ornithology: the branch of biology that deals with the study of birds

Thecodonts: extinct reptiles that lived during the Permian period; they were the ancestors of both dinosaurs and birds

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