What Makes a Bird a Bird?

What Makes a Bird

What is a bird? Birds are vertebrates with feathers. They have distinctive bills, are endothermic, produce large eggs, have elaborate parental behavior, and have extraordinary vocal abilities.

The anatomy and physiology of most birds are adapted for flight. Birds share with reptiles many anatomical features that distinguish them from mammals.

What Makes a Bird a Bird?

Fishes, reptiles, and mammals are also vertebrates; however, birds are unique in having the following features:

  • Birds have feathers, which are modifications of the outer skin. Birds are the only vertebrate with feathers.
  • The ability to fly is perhaps the most common attribute of birds. Sustained flight is not unique to birds; bats and insects are also capable of flying. What sets birds apart is having morphological and physiological adaptations to flight. Birds are capable of extraordinary navigational and long-distance migrations and maneuverability exploits.
  • Feathers are essential for both temperature regulation and flight. They insulate the body and help maintain a high body temperature.
  • Birds have a bill. Although birds’ bill comes in a wide variety of lengths, shapes, and sizes, their presence immediately identifies the animal possessing one as a bird. Even people with the least interest in the natural world can associate a bill with a bird.
  • Birds have a single occipital condyle on the back of the skull, a single middle-ear bone, and nucleated red blood cells.

Morphological Adaptations of Birds

  • Fused bones of the pelvis, feet, hands, and head contribute to the body’s rigidity and strength. Horizontal, backward-curved projections on ribs, called uncinate processes, overlap other ribs and strengthen the thorax walls.
  • The furcula, or wishbone, is uniquely avian, evolving as a central element of the earliest known birds’ flight apparatus. The furcula prevents lateral compression of the chest during the downstroke of the wings. The furcula also is an anchoring site for the pectoral flight muscles.
  • The wing itself is a highly modified forelimb that is nearly incapable of functions other than flight, with a few remarkable exceptions.
  • Birds’ bones are lightweight structures, spongy, strutted, or hollow.
  • Modern birds also lack teeth and the associated heavy maxillary bones of the jaw.

What Makes a Bird
Adaptations to extreme conditions. The Coastal Miner (Geosita peruviana) (left) lives in the very dry Atacama Desert, while the Snow Bunting (Plectrophenax nivalis) thrives in extremely cold environments. Photo of Bunting: Eric B├ęgin/Flickr/CC by 2.0

Birds Are Endothermic

Endothermic? It means that they generate and maintain high body temperature from metabolic heat production. Endothermy allows birds to thrive in the most extreme habitats and range of ambient temperatures.

Being endothermic allows birds to fulfill the metabolic demands of flight and temperature regulation. The red fibers of avian flight muscles have an extraordinary capacity for sustained work and produce heat by shivering.

The avian circulatory and respiratory systems are powerful and efficient in delivering fuel and removing waste and metabolism for flight.

Birds have a unique Reproductive System

The reproductive systems of birds are also unique. They produce large, richly provisioned external eggs, the most elaborate reproductive cells of any animal. But that is not all.

Birds nurture the growth of the embryos and the young. After the eggs hatch, the young require dedicated parental care.

As part of their reproductive behavior,  birds have elaborate mating systems, nesting behavior, territoriality, brood parasitism, coloniality, and a suite of other social adaptations.

Sensory Systems

While still poorly understood, birds have highly developed neural systems and acute senses that allow them incredible navigation and communication acts.

In particular, the extraordinary vocal abilities of birds have few parallels among animals. Only human vocal production is comparable.

Are birds nothing but Glorified Reptiles?

To some extent, this is true. Birds have evolved from reptiles and share many characteristics. In many regards, birds are structurally similar to reptiles.

Conversely, birds have dramatic differences, such as having feathers, flying, being endothermic, or being capable of generating their own heat and regulating their body temperature.

Similarities Between Birds and Reptiles

Shared characteristics between birds and reptiles include:

  • The skulls of both articulate with the first neck vertebra through a single ball-and-socket device, the occipital condyle (mammals have two condyles).
  • Birds and reptiles have a simple middle ear with only one ear bone; the stapes (mammals have three middle-ear bones).
  • The lower jaws, or mandibles, of both birds and reptiles, have five or six bones on each side (mammals have only one mandibular bone, the dentary).
  • Avian and reptilian ankles are sited in the tarsal bones, not between the tibia and tarsi as in mammals. The scales on the legs of birds are similar to the body scales of reptiles.
  • Both birds and reptiles lay a yolked (polylecithal), polar (telolecithal) egg in which the embryo develops on the surface by shallow (meroblastic) divisions of the cytoplasm on the surface of the egg. 
  • Female birds and some female reptiles have the XY (heterogametic) sex chromosome combination (in mammals, males are the heterogametic sex). Birds and reptiles have nucleated red blood cells (the red blood cells of mammals lack nuclei).
bird diversity

Why So Many Bird Species?

The evolution of birds from a reptilian ancestor and their subsequent diversification is the result of speciation.

What is speciation? In a rather simple form, speciation is the splitting of one species into two with independent evolutionary futures and dissimilar habits.

What is Adaptive Radiation? The splitting of many species that adapt to many somewhat different ecological setting is called adaptive radiation. The process of adaptive radiation moves along with variations in form and function that enable species to exploit their environment.

So Many Bird Species

Bill sizes and shapes change with the types of food eaten; leg lengths vary with habits of perching or terrestrial locomotion, and wing shapes change with types or patterns of flight and aerodynamic efficiency.

Birds range in size from 2 grams (hummingbird) to 100,000 grams (ostrich). The largest birds, such as ostriches and rheas, are so big that they cannot fly.

Ecological Flexibility

Modern birds reflect adaptive radiation in countless forms and functions of their body parts. Plantcutters and Hoatzins eat leaves and buds, while finches and tanagers eat seeds and fruit. Flycatchers have an array of bill shapes to catch insects stationary or on the wing.

Some have taken to the water to consume fish at various depths. Mergansers and grebes dive for fish and invertebrates found at varying distances from the surface. Kingfishers have a large bill and little feet to plunge into the water to catch fish near the surface.

At higher trophic levels, hawks and falcons have developed strong claws and bills to catch, kill, and eat live animals, including other birds.

At the top of the trophic chain are the vultures and condors that have specialized in eating carrion.

Few birds are specialized herbivores. Mammals have occupied most of the grazing and browsing niches.

bird beaks and function

Diversity of bills and Diets

A diversity of diets correspond to an assortment of bills. An illustration of the variety of bill forms that can evolve during adaptive radiation is those of the Galapagos finches.

These finches evolved from finches that strayed out over the Pacific Ocean from mainland South America millions of years ago. The finches made landfall on one of the Galapagos Islands, then flourished and spread throughout the archipelago.

Isolated populations changed in genetic composition and appearance, at first imperceptibly and then conspicuously. Subtle changes in bill shapes and sizes led to a proliferation of bill types and feeding ecologies to match the available ecological niches.


Various forms of locomotion further expand the ecological flexibility of birds. There are specialized flying birds such as swifts and swallows and specialized swimmers, runners, waders, climbers, and perchers.

Shorebirds illustrate the diverse type of locomotion to procure food. Some shorebirds are aerial pirates; others are wading, and others are diving species.

Birds soar through the sky, dash, and stride across the land, hop actively from branch to branch, hitch up tree trunks, and swim powerfully to great depths in the sea.

The combination of forelimbs adapted for flight and hindlimbs for bipedal locomotion gives birds a tremendous range of ecological options.

bird feet
The feet of songbirds (a) are adapted for perching on branches. Raptors have feet with large claws to grab their prey (b). Ducks’ feet are suitable for swimming (c), while an ostrich’s feet are suitable for running. 

Wing shapes

Wing shapes and modes of flight differ significantly, from the long, narrow wings of the albatross, adapted for soaring over the oceans, to the short, round wings of wrens, adapted for agile fluttering through dense vegetation.

At another extreme are the adaptations of wing-propelled diving birds, such as penguins, which use their wings as flippers.

Related: Wing or Flight Feathers.

The feet and legs of birds

Like the structure of bills and wings, feet and legs’ anatomy tells us much about avian ecology. At one extreme are the tiny, weak feet and short legs of specialized aerial species such as tropicbirds and swifts.

At the other extreme are the long, powerful legs of wading and cursorial birds such as storks and ostriches.

The long toes of herons and jacanas, which spread the bird’s weight over a large surface area, facilitate walking on soft surfaces. Sandgrouse can dart on soft desert sands, and ptarmigan can walk on snow by virtue of snowshoe-like adaptations of their feet.

Lobes on the toes of rails and webbing between the toes of flamingos and avocets reduce sinking into soft mud.

Climbing birds such as woodpeckers have large, sharply curved claws, and nuthatches, and other birds that climb downwards have prominent hind toes with a large claw.

Morphology of the Avian Foot

The foot bones (three tarsals) are fused to the metatarsals, creating an elongate, strong single element, the tarsometatarsus, which enables birds to walk on their toes rather than on the whole foot.

What appears at first glance to be a backward-bending knee joint is really the ankle joint. Legs, as well as feet, are adapted for various modes of locomotion.

Whereas arboreal birds have short legs that aid balance on unstable twigs, terrestrial birds have long legs. Ostriches, which have long, strong legs and only two toes, are highly adapted for running using long strides.

Conversely, sandpipers dash on short, thin legs. Short, heavy legs and big feet tend to evolve in large, flightless herbivorous birds such as moas and the dodo on islands without predators.

Bird Adaptations to life in Trees

Arboreal, or tree-dwelling, species, which constitute most birds, have feet designed for gripping branches tightly. Among the features of such feet is a long tendon that passes around the backside of the ankle joint.

When a bird bends the joint to squat, the tendon automatically flexes, locking the toes around the branch. When a bird stands, the tension and the toe’s grasp relax.

The foot of the dominant group of perching birds, the songbirds, or Passeriformes, is perhaps the most advanced in this respect. The large, opposable single rear toe (hallux), which enhances the ability to grip a branch, is unusual among vertebrates.

The tendons and muscles that flex the toes are arranged to facilitate perching at the expense of control of individual toe positions. A special system of ridges and pads between the tendons that bend the toes and the toe pads’ insides acts as a natural locking mechanism, allowing birds to sleep while perching.

Perching birds
The feet of the White-capped Tanager Sericossypha albocristata are adapted for life in trees. Photo Sericossypha on branches.  

Center of Gravity

A bird’s center of gravity must remain over and between its feet, particularly when it perches, squats, or rises.

Equal lengths of the two main leg bones (tibiotarsus and tarsometatarsus) of long-legged birds ensure this relationship. Therefore, unlike most swift quadrupeds, birds cannot increase running ability by evolving distal leg length.


Foot-propelled diving birds such as loons and grebes have sacrificed balance on land for swimming ability. They have powerful legs situated at the rear of a streamlined body.

Even more, articulating with their long, narrow pelvis is a short femur, a long tibiotarsus with an extension (cnemial crest) for muscle attachments, and a laterally compressed tarsometatarsus that reduces resistance in the water.

Final Remarks:

Birds are characterized as vertebrates with feathers. They have distinctive bills, are endothermic, produce large external eggs, and have elaborate parental behavior. Perhaps one of the more distinctive aspects of birds is that they have extraordinary vocal abilities.

The anatomy and physiology of most birds are adapted for flight. Birds share with reptiles many anatomical features that distinguish them from mammals.

The process of adaptive radiation, illustrated by the Galapagos finches, shows how bill forms can evolve with ecological opportunities.

  Entries of Interest


  • Attenborough, D.1998 The Life of Birds.
  • del Hoyo, Josep; Andy Elliott; Jordi Sargatal (1992). Handbook of Birds of the World.
  • Gill, Frank (1995). Ornithology. New York: W.H. Freeman.
  • Luiggi, Christina (July 2011). “On the Origin of Birds.” The Scientist.