Birds are warm-blooded vertebrates characterized by feathers on their bodies, toothless beaked jaws, hard-shelled calcareous eggs, and a four-chambered heart with a high rate of metabolism. They are experts at flight (except flightless birds, like ostrich, penguins, and some diverse endemic species living in islands), equipped with wings, which are nothing but forelimbs modified for an aerial lifestyle. Their bones are lightweight and pneumatic, bearing air cavities connected to the lungs.

They belong to the class Aves under the tetrapod clade Archosauria, including their sister group, crocodilians. They have a worldwide distribution with an estimated population of around 11,000 known living species. They are extremely social, using tuneful songs as calls to attract potential mates or communicate with their conspecifics.



While the smallest living bird, the bee hummingbird, measures 6.3 cm (2.5 inches) in length and weighs less than 3 gm (about 0.1 ounces), the largest living bird, the common ostrich, stands over 9 feet (2.75 meters) tall and weighs 150 kg (330 lbs).

Body Plan

The avian skeleton is characterized by lightweight bones filled with air cavities (pneumatic bones) directly connected to the respiratory system. Their forelimbs are modified into wings by the fusion of bones, reduction of digits and claws, and inclusion of attachment points for the feathers. The hindlimbs culminate in claws and are covered with scales to help them perch on tree branches.

The skull is large in adults, and all bones are fused, leaving no visible sutures. The vertebral column is divided into cervical, thoracic, lumbar, and caudal regions, with remarkable variation in the number of vertebrae in the cervical region. They have flattened ribs that fuse to a keeled sternum, providing attachment sites for the flight muscles.

Plumage and Scales

Birds have feathers on their bodies, which are nothing but epidermal growths attached to the skin. They are present in specific skin tracts called pterylae and serve different purposes apart from flight, such as thermoregulation, camouflage, and attracting mates.

They shed their feathers annually and reacquire them through molting, though many birds may molt multiple times yearly. Ducks and geese lose all their feathers at once, temporarily becoming flightless.

Birds also have scales (homologous to those of reptiles and mammals) on specific portions of their bodies, such as the toes and metatarsus. These scales are clustered keratin, like beaks, claws, and spurs. Though non-overlapping in most birds, kingfishers and woodpeckers have overlapping scales.

Organ System


Birds have a four-chambered, myogenic (muscle-driven) heart covered by a fibrous pericardial sac. The atrium and ventricle of each side are separated by atrioventricular valves that prevent blood from flowing backward. Like a mammalian heart, the avian heart wall has three layers: epicardium (outer layer), myocardium (middle layer), and endocardium (inner layer).

The sinoatrial node sets the heart’s pace, using calcium to send a depolarizing signal from the atrium through the right and left atrioventricular bundle, which leads to the contraction of the two ventricles.

The arteries that carry oxygenated blood throughout the body comprise thick, elastic muscles that contract (vasoconstriction) while the blood flows into the capillaries of the different organs. The gaseous exchange occurs at these capillaries, following which the deoxygenated blood travels through the veins by dilating them (vasodilation). After the blood reaches the right atrium, it moves to the right ventricle, passing it to the lungs for oxygenation.


Birds have a characteristically unique respiratory system adapted for their aerial lifestyle. Their lungs are connected to several pairs of nonvascular air sacs, which, in turn, are connected to the air spaces in the bones.

When the bird inhales air, most of it bypasses the lungs and flows straight into the air sacs, filling the bones with sufficient air. On exhalation, the used air moves out of the lungs while the air stored in the posterior sacs is forced into the lungs, thus ensuring a constant supply of fresh air.


The avian digestive tract is characterized by the presence of a crop, an enlargement of the esophagus, and the muscular gizzard of the stomach. While the former stores food, the latter contains swallowed stones that substitute teeth and help grind food.


They excrete nitrogenous waste in the form of uric acid (uricotelic) using kidneys. However, some birds, like hummingbirds, are facultatively ammonotelic, releasing waste as ammonia, whereas a few others expel creatine.

Most birds, except ostriches, lack a urinary bladder or urethral opening and release nitrogenous waste as semi-solid waste along with feces.


The brain is the central organ of a bird’s nervous system. It is large and divided into the forebrain, midbrain, and hindbrain, each performing a different function. While the cerebrum in the forebrain controls most behavior patterns, the cerebellum coordinates the movement of different body parts. To the posterior of the cerebellum lies the hindbrain that connects the rest of the brain to the spinal cord.

Birds have a well-developed visual system, marked by large, tetrachromatic eyes capable of detecting colors that most animals lack. They possess ultraviolet (UV) sensitive cone cells for sensing colors; however, they sometimes contain double cones that enable achromatic vision. While birds with eyes on the sides of their heads have wide vision, those with eyes on the front of their heads, like owls, have binocular vision characterized by acute depth estimation.

The eyes are covered by a thin nictitating membrane (third eyelid) that moves horizontally and keeps them moist. A fan-like structure called the pecten constantly supplies blood to the innermost retina.


Male birds have two testes, and those belonging to the groups Palaeognathae (except kiwis), Anseriformes (except screamers), and Galliformes have a specific copulatory organ called the penis. However, birds in the group Neoaves completely lack this organ.

Females usually have a single ovary and an oviduct, but many species, like kiwis, have two functional ovaries. They also have sperm-storing tubules to preserve the gametes long after copulation.


Aves and their sister group, Crocodilia, are the only living representatives of the reptile clade Archosauria. Scientists Gauthier and de Queiroz described ‘Aves’ in four ways:

  1. All archosaurs that are more closely related to birds than to crocodiles (alternately Avemetatarsalia)
  2. All advanced archosaurs with feathers (alternately Avifilopluma)
  3. All feathered dinosaurs that fly (alternately Avialae)
  4. The last common ancestor of all the currently living birds and all its descendants (a crown group)

All modern birds belong to the crown group Aves, which has two infraclasses: the Palaeognathae, which includes the flightless ratites and the weak-flying tinamous, and the most diverse Neognathae, containing all other species.

The following list contains all the extant orders of birds classified under the two infraclasses.

Birds (Aves)



Based on fossil records, birds evolved from theropod dinosaurs, specifically those in the clade Maniraptora, which included feathered dinosaurs like dromaeosaurids and oviraptorosaurs. Some members, like those belonging to the genus Microraptor, probably had specific physical features that enabled them to glide or fly.

The earliest avialian fossil dates back to the Late Jurassic Period (about 160 million years ago), while the most well-known fossil, Archaeopteryx, originated slightly later (about 155 million years old). The Archaeopteryx possessed enlarged claws on the second toe, long feathers or “hind wings” covering the hind limbs and feet, and other characteristics that establish it as a missing link between reptiles and birds.

Early Evolution

Avialians started evolving during the Cretaceous Period and underwent multiple changes that proved seminal in the evolution of birds. They lost their clawed wings, and their tails became increasingly stiff, culminating in the appearance of the fused vertebra, the pygostyle. In the Late Cretaceous (about 100 million years ago), the ancestors of all modern birds developed a broad pelvis to lay larger eggs, while around 95 million years ago, they evolved a better sense of smell. With time, they developed an enlarged, keeled sternum, the alula on their wings, and gradually lost the grasping power of their limbs.


The first diverse lineage to have evolved was the Enantiornithes, characterized by the reverse construction of their shoulders compared to modern birds. The ones to follow were the second major avialan lineage, the Euornithes, including semi-aquatic avialians, such as shorebird-like species, waders, and diving species. They were also the first avialians to have developed a moveable fan of tail feathers, probably as a replacement for the hindlimbs.

Fossils like those of the genus Vegavis reveal that the diversification of modern birds started before the Cenozoic Era. Asteriornis from the Maastrichtian Age is believed to be a close relative of Galloanserae, the earliest diverging lineage within Neognathae.

Recent research (2015) suggests that modern birds originated early in the Late Cretaceous, probably in Western Gondwanaland. However, all the major groups diversified only after the Cretaceous–Palaeogene extinction event that killed all non-avian dinosaurs. The formation of land bridges between South America and North America played a crucial role in the expansion and diversification of birds in the Arctic and the tropics.

Distribution and Habitat

Although birds are abundant in almost every terrestrial habitat on all continents, their highest diversity is in the tropics. Birds like snow petrels extend their breeding colonies as far south as 440 km inland from Antarctica.

Many species, such as the ring-necked pheasant, have been purposely introduced by humans in certain areas as game birds, eventually establishing stable populations. In contrast, monk parakeets have accidentally entered several North American cities and heavily populated them.


Birds mainly feed on nectar, fruits, plants, seeds, and a variety of small animals, such as insects and other bird species, that hop onto them from branches. Scavenging birds, such as vultures and hawks, feed on dead and decaying remains.

Those that feed entirely on one specific food type, such as hummingbirds feeding on nectar, are specialists, whereas pigeons, sparrows, and crows are generalists living off multiple food items.


While most birds are active during daytime (diurnal), owls and nightjars are active at night (nocturnal) or during twilight (crepuscular).


Feather Care


Many land birds, waterbirds, and shorebirds set out to journey across long distances to avoid extreme weather conditions and optimize the availability of resources and breeding habitats. While land birds have a flight range of around 2,500 km, shorebirds can cover distances as long as 4,000 km. The peregrine falcon can reach more than 300 km (186 miles) per hour, making it the world’s fastest bird.

The longest annual migrations are recorded in muttonbirds, which spend their summers in the North Pacific off Japan, Alaska, and California and move southwards to New Zealand and Chile, making for a year-round trip of 64,000 km.



Birds have developed several mechanisms to rest or sleep (roost) when they have some respite from foraging.


Birds communicate through songs and calls produced by their vocal organ, the syrinx. These sounds help identify individuals, evaluate potential mates and attract them, claim territories, and warn members of groups. Some songbirds can simultaneously use both sides of their syrinx to produce different acoustic effects.

Palm cockatoos and woodpeckers drum on the surface of the wood and use such mechanical sounds for communication.


Though some birds live in small, incidental, or family groups, others flock in large numbers for safety and efficient foraging. They also form associations with non-avian species, like dolphins and tuna, pushing shoaling fish toward the water’s surface, which gives them access to birds. Hornbills also form a mutualistic association with the common dwarf mongoose, where they forage together and warn each other of nearby predators, like birds of prey.


While the ruby-throated hummingbird lives for only three to five years, the world’s oldest known wild bird, a Laysan Albatross, is around 73 years old.


The majority of birds are monogamous or have exclusive partnerships throughout their lives; however, in birds belonging to the family Anatidae, extra-pair copulation or infidelity is common. Other mating systems, like polygamy, polyandry, polygyny, and polygynandry, are also found across the different families of birds.

Sexual selection

Male birds engage in some courtship displays to woo their mates. These typically involve singing, but sometimes, the males drum their tails, dance, or take elaborate aerial flights to attract the females. Usually, the females select visually appealing males, but in phalaropes, it is the opposite, where the males choose vibrant females as their mates. Some birds, such as house sparrows, acorn woodpeckers, ravens, and ostrich, are among the 130 species of birds that display homosexual behavior by involving in sexual interactions with individuals of the same sex.


Most female birds develop a special brood pouch by shedding feathers close to the belly and getting ready to lay and incubate eggs. They lay amniotic eggs, with a fluid-filled amniotic membrane covering the embryo. These eggs are covered by hard calcareous shells composed of calcium carbonate, which are often patterned to help camouflage against their backgrounds.

Eggs are usually laid in cup-shaped nests built using leaves, twigs, sticks, lichen, and other natural components. However, most birds that live in open habitats, like the common guillemot, lay eggs on bare ground. Similarly, emperor penguins keep their eggs protected between their body and feet. Sometimes, birds, like cuckoos and honeyguides, lay their eggs in another bird’s nest (brood parasitism) so that the host accepts and raises their young.

The female or both parents incubate the eggs using brood patches, through which heat passes. The incubation period may vary from 10 days in woodpeckers, cuckoos, and perching birds to more than 80 days in kiwis and albatrosses. Some megapodes incubate their eggs using the warmth of sun rays, decaying vegetation, or volcanic sources.

Parental Care

In over 90% of bird species, both parents care for the chicks. In crows, Australasian wrens, and Australian magpies, the breeding pair’s close relatives often care for the young (alloparenting). Many seabirds, such as the great frigatebird, continue to provide parental care for up to six months till the young are ready to fledge. Their young are also fed for an additional 14 months after fledging.


Ecological Importance

Conservation Status

Although hundreds of birds have gone extinct in the past, the worst avian extinction occurred during the colonization of Melanesian, Polynesian, and Micronesian islands, washing away around 750 to 1,800 species.

The most common anthropogenic threat to bird diversity is the constant degradation and loss of natural habitats. Other threats include long-line fishing bycatch, overhunting, competition from invasive species, and climate change.

With their populations constantly declining, 1,227 bird species were listed as threatened by BirdLife International and the IUCN in 2009. However, conservation efforts are being undertaken to restore existing habitats and breed captive populations for reintroduction in the wild.

References Article last updated on 9th May 2024

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