Bird Anatomy: Interactive 3D Pigeon with Labeled Diagrams

A bird is a feathered, warm-blooded vertebrate with a toothless beak, front limbs turned into wings, a light skeleton, a crop and two-part stomach instead of teeth, small rigid lungs ventilated by air sacs, a four-chambered heart, and a single cloaca for droppings, urine and eggs. Most of its body plan is a trade-off between flying and carrying weight: heavy parts, such as the gizzard and the flight muscles, sit close to the centre of the body.

This page describes the rock pigeon (also called the rock dove or feral pigeon), Columba livia. Wild-type birds are 30–35 cm long with a wingspan of 62–68 cm and weigh about 240–380 g. They are blue-grey with an iridescent green and purple neck, two black bars on each wing and a white lower back. Pigeons are the standard bird of zoology labs; the chicken, covered on its own page, differs in several organs that are noted below.

For the bones, see the bird skeleton page; for a step-by-step lab, the pigeon dissection guide.

Bird topography divides the body into named regions so plumage can be described exactly. On the head: forehead, crown, nape, lores (between eye and bill), cheek, chin and throat. On the body: mantle and back, rump, breast, belly, flanks and vent. On the wing: lesser, median and greater coverts, the alula, primaries, secondaries and tertials. The tail has rectrices with upper- and undertail coverts, and the legs end in a bare, scaly tarsus and toes.

• Bill (beak): The toothless bill: a horny keratin sheath (rhamphotheca) over the bones of the upper and lower jaws. Picks up seeds and grit, drinks (pigeons can suck water up without tipping the head back), preens feathers and feeds the squabs.
• Cere (operculum): The soft, swollen, whitish patch of skin at the base of the upper bill, over the nostrils. Covers and partly shields the nostrils.
• Nostrils (external nares): Slit-like openings into the nasal cavity, at the base of the upper bill under the cere. Let air into the nasal cavity, which opens into the mouth through a slit in the palate (the choana).
• Eye: A very large eye on each side of the head, with an orange iris in the rock pigeon. Wide field of view for spotting predators; good colour vision.
• Ear opening (external acoustic meatus): A small opening behind and below the eye, hidden under feathers (ear coverts). Leads to the eardrum and middle ear; birds have no external ear flap (pinna).
• Uropygial (preen) gland: A small skin gland on the back at the base of the tail that secretes preen oil. The bird squeezes oil from it with the bill and spreads it through the plumage while preening.
• Feet and scales (podotheca): The bare, red, scaly skin of the tarsus and the four toes. Support the bird while perching and walking; the scales are keratin, like the bill.
• Vent (cloacal opening): The single external opening of the cloaca, under the base of the tail. Releases droppings (faeces plus white urates), and eggs or sperm.

Pigeons show a few features that students often miss. The soft white swelling at the base of the upper bill is the cere (operculum), small in wild rock pigeons and much larger in many domestic breeds. The ear opening is hidden under the feathers behind the eye; birds have no external ear flap. And the preen gland at the base of the tail is small in the rock pigeon, and Columbidae is one of the families in which some species lack it entirely.

The bird digestive system runs, in order, from the bill and mouth through the oesophagus, crop, proventriculus (glandular stomach), gizzard (muscular stomach), duodenum, small intestine, caeca and rectum to the cloaca, which opens at the vent. The liver and pancreas empty into the duodenum. Birds have no teeth: food is swallowed whole, stored in the crop, softened by acid and enzymes in the proventriculus and ground up in the gizzard.

• Oesophagus (esophagus): The muscular food tube from the pharynx down the neck to the crop and on to the stomach. Carries swallowed food by peristalsis; its folded wall stretches to take whole seeds.
• Crop (ingluvies): A thin-walled, two-lobed pouch of the oesophagus at the base of the neck. Stores and softens food; during breeding its lining produces crop milk for the squabs.
• Proventriculus (glandular stomach): The first, spindle-shaped part of the stomach, with a thick glandular lining. Secretes hydrochloric acid and pepsinogen that start the digestion of protein.
• Gizzard (ventriculus): The round, very muscular second stomach that grinds food with swallowed grit. Crushes seeds against grit, acting as the bird's teeth.
• Duodenum (duodenal loop): A U-shaped loop of small intestine leaving the gizzard, with the pancreas held inside it. Receives bile and pancreatic juice and begins most chemical digestion.
• Pancreas: A thin, pale gland lying between the two limbs of the duodenal loop. Makes digestive enzymes (and hormones such as insulin and glucagon).
• Liver: A large, dark red-brown organ with two lobes. Makes bile, stores glycogen and processes absorbed nutrients.
• Small intestine (jejunum and ileum): The long, coiled middle part of the intestine after the duodenal loop. Completes digestion and absorbs nutrients.
• Caeca: A pair of tiny blind pouches where the small intestine joins the rectum. In pigeons they are vestigial and contain lymphoid tissue; in chickens and grouse they are long and ferment plant fibre.
• Rectum (colon): The short, straight large intestine leading to the cloaca. Absorbs water and salts, including water returned backward from the urine.

The cloaca is divided into three chambers. The coprodeum receives faeces from the rectum, the urodeum receives the ureters and the reproductive ducts, and the proctodeumopens to the outside through the vent. Bird droppings are therefore a mixture: dark faeces capped with white uric acid (urates) from the kidneys.

Birds and mammals digest food with the same chemistry, but birds replace teeth with a gizzard, add a crop for storage, split the stomach in two, and end the tract in a cloaca.

A bird's digestive system matches its diet. Seed-eaters such as pigeons have a large crop, a very thick, muscular gizzard with a hard koilin lining, and swallow grit that the gizzard uses as millstones. Birds of prey and owls eat meat, have thinner-walled gizzards, and cast pellets: the gizzard compacts fur, feathers, bones and claws into a pellet that the bird coughs up, within about six to ten hours of a meal.

Caeca vary in the same way. Grouse and chickens have long paired caeca that ferment plant fibre; in pigeons the caeca are vestigial and contain lymphoid (immune) tissue. Pellets are cast by many other birds too, including herons, gulls, kingfishers, crows and most shorebirds.

A feather has a hollow base, the calamus, held in a follicle in the skin, and a shaft, the rachis, carrying two vanes. Each vane is a row of barbs. Each barb carries two rows of barbules: the barbules facing the feather tip carry tiny hooks (hooklets, hamuli or barbicels) that catch the grooved barbules of the next barb, zipping the vane into a flat, airtight surface.

The barbs near the base of most contour feathers are fluffy, plumulaceous barbs without interlocking hooks; they trap air for warmth. Many contour feathers also have a small downy afterfeather attached near the calamus. Under a microscope, the nodes and pigment along downy barbules differ between bird groups, which is how Smithsonian scientists identify birds from feather fragments after aircraft bird strikes.

The flight feathers of the wing (remiges) are the primaries on the hand, the secondaries on the forearm and the tertials near the body. Rows of shorter coverts cover their bases, and the alula sits on the thumb. The tail feathers (rectrices) fan out from the pygostyle.

• Primary flight feathers (primaries): The long outer flight feathers of the wing, attached to the bones of the hand. Produce thrust on the downstroke and shape the wingtip; they can spread like fingers or close.
• Secondary flight feathers (secondaries): The flight feathers of the forearm, attached to the ulna. Form the lift-producing inner wing surface.
• Tertials: The innermost long wing feathers, beside the body, over the upper arm. Close the gap between wing and body and cover the folded secondaries.
• Wing coverts: Rows of short feathers that cover the bases of the flight feathers and the leading edge of the wing. Smooth the airflow over the wing and protect the bones and patagium (wing skin fold) underneath.
• Alula (bastard wing): A small tuft of stiff feathers on the alular digit (the 'thumb') at the front of the wrist. Lifted like an aircraft's leading-edge slat, it keeps airflow attached at slow speeds and steep angles, as in landing.
• Tail feathers (rectrices): The long, paired flight feathers of the tail, rooted around the pygostyle. Steer, brake and add lift; the tail spreads wide on landing.

Most non-passerine birds, including pigeons, have 9–11 primaries; secondary counts range from about six in hummingbirds to around 40 in some albatrosses. Most birds have 12 rectrices (six pairs), but centuries of breeding have made the tail-feather count highly variable in domestic pigeons.

A blood feather (pin feather) is a new feather that is still growing. It develops inside a keratin sheath, with a blood supply running up its shaft. As it grows, the tip matures first and the blood withdraws towards the follicle; once the feather is full-grown its blood supply stops and the bird preens away the sheath. A broken blood feather can bleed heavily, so birds in molt are handled gently.

Molt is the regular replacement of feathers. In most passerines the wing molt starts between the innermost primary and the first secondary and spreads in both directions, a few feathers at a time, so the bird can keep flying. Some waterbirds, such as most ducks, loons and grebes, drop all their flight feathers at once and are flightless for three to four weeks.

Birds breathe with small, rigid lungs and a system of thin-walled air sacs. The sacs work as bellows; the lungs, which barely change volume, only exchange gas. Air flows through the lungs' parabronchi in the same direction during both inspiration and expiration, which is why bird lungs extract oxygen so efficiently. Birds have no diaphragm.

• Trachea (windpipe): A long tube of complete, overlapping cartilage rings running down the neck. Carries air between the larynx and the syrinx and bronchi.
• Syrinx: The bird's voice box, at the fork of the trachea into the two bronchi. Vibrating tympaniform membranes in its walls produce the pigeon's coo.
• Lungs: Small, bright pink, spongy lungs pressed into the ribs on the roof of the thorax. Gas exchange in the air capillaries around the parabronchi. The lungs barely change volume: the air sacs do the pumping.
• Cervical air sacs: A pair of small air sacs at the base of the neck, above the lungs. Part of the anterior air sac group; their extensions pneumatise the neck and thoracic vertebrae.
• Clavicular air sac: A single air sac at the thoracic inlet, around the syrinx and the base of the heart. Sends diverticula into the humerus and around the shoulder, and surrounds the syrinx.
• Cranial thoracic air sacs: A pair of air sacs on the sides of the thorax, ventral to the lungs. Anterior air sacs: they receive air that has already passed through the lungs and send it out on expiration.
• Caudal thoracic air sacs: A pair of air sacs behind the cranial thoracic sacs, on the sides of the body cavity. Posterior air sacs: they fill with fresh air on inspiration and push it through the lungs on expiration.
• Abdominal air sacs: The largest air sacs, filling spaces among the intestines in the rear abdomen. Posterior air sacs that store fresh air; their extensions reach into the synsacrum and around the hip sockets.

Air sacs also reach into the bones. In pigeons and chickens the clavicular sac sends a diverticulum into the humerus, the cervical sacs extend along the vertebrae, and the abdominal sacs reach into the synsacrum and around the hip sockets. Sound, finally, comes not from the larynx but from the syrinx, deep in the chest where the trachea divides, surrounded by the clavicular air sac.

Birds have a four-chambered heart: two atria and two ventricles, with oxygen-rich and oxygen-poor blood kept fully separate, as in mammals. The bird heart is large for the body size, and its aorta curves to the right instead of the left.

• Right atrium: The thin-walled chamber that receives oxygen-poor blood from the body. Receives blood from the two cranial venae cavae and the caudal vena cava and passes it to the right ventricle.
• Left atrium: The chamber that receives oxygen-rich blood from the lungs. Passes blood from the pulmonary veins to the left ventricle.
• Right ventricle: The crescent-shaped chamber wrapped around the right side of the left ventricle. Pumps oxygen-poor blood to the lungs through the pulmonary arteries.
• Left ventricle: The thick-walled, cone-shaped main pumping chamber forming the heart's apex. Pumps oxygen-rich blood into the aorta for the whole body.
• Right aortic arch and dorsal aorta: The main artery: it leaves the left ventricle, arches over to the right, and runs back under the spine. Carries oxygen-rich blood to the body.
• Brachiocephalic arteries: Two large arteries from the aortic arch, each splitting into a common carotid and a subclavian artery. Supply the head and neck (carotids) and the wing and huge breast muscles (subclavian, pectoral arteries).
• Venae cavae: The great veins returning oxygen-poor blood to the right atrium. Return blood from the body to the heart.

Blood returns to the right atrium through three great veins (two cranial venae cavae and one caudal vena cava), passes to the right ventricle and the lungs, returns to the left atrium and is pumped by the thick left ventricle into the right aortic arch. Two large brachiocephalic arteries leave the arch for the head and the wings; their pectoral branches feed the flight muscles.

Bird kidneys are three-lobed and pressed into the pelvis; their ureters run straight to the cloaca, because birds have no urinary bladder. Males have two internal testes; females of most species, including pigeons, have only a left ovary and left oviduct.

Switch the 3D model to Female or Male to compare the organs.

The pigeon brain has two smooth cerebral hemispheres, a folded cerebellum, large optic lobes on its lower sides and a medulla that leads into the spinal cord. The size of the optic lobes reflects how much the pigeon relies on vision.

• Cerebral hemispheres: The two smooth, unfolded hemispheres at the front of the brain. Learning, memory, navigation and complex behaviour.
• Optic lobes (optic tectum): Two large rounded swellings on the lower sides of the brain. Process vision, the pigeon's dominant sense.
• Cerebellum: A single, ridged (folded) lobe at the back of the brain. Coordinates balance and movement, crucial in flight.
• Medulla oblongata: The base of the brain, tapering into the spinal cord. Controls breathing, heart rate and other automatic functions.

Bird eyes have three eyelids, the third being the nictitating membrane that sweeps across from the front corner, and inside, a pleated, blood-rich comb called the pecten. Each eye is stiffened by a ring of bony plates, the sclerotic ring.

The largest muscles of a pigeon are the two flight muscles on the breast. The pectoralis pulls the wing down; the supracoracoideus, underneath it, lifts the wing using a tendon that runs through a bony pulley at the shoulder.

• Pectoralis (breast muscle): The huge, dark red, superficial breast muscle that powers the downstroke. Pulls the wing down and forward: the power stroke of flight.
• Supracoracoideus: The deep breast muscle, under the pectoralis, that lifts the wing through a pulley at the shoulder. Raises the wing and twists (supinates) the humerus during the upstroke.
• Thigh and drumstick muscles: The muscles around the femur and tibiotarsus that move the leg and toes. Walk, perch and launch the bird into the air; long tendons run down to the toes.

This is the text version of the 3D model: every labeled structure of the rock pigeon, grouped by body system.

Pigeons and doves are unusual among birds in feeding their young a secretion from the crop. In both males and females, the hormone prolactin makes the lining of the crop thicken during brooding; its surface cells fill with fat and are shed into the crop as a thick, protein- and fat-rich fluid that also carries immunoglobulins. During production the crop lining becomes visibly congested and thickened. (Crespo et al. (2018): Galliformes and Columbiformes, Pathology of Wildlife and Zoo Animals.)

The anatomy supports this function. In domestic pigeons the crop is a large, thin-walled sac with two lateral diverticula at the entrance to the thorax, just in front of the breast muscles. Squabs start on crop milk alone; over roughly 4–7 days the parents switch to feeding them mostly regurgitated seed. Crop milk is rich in fat and protein and contains essentially no carbohydrate. Flamingos and male emperor penguins also produce a crop or oesophageal milk, an example of the same solution evolving more than once. (Wikipedia: crop milk.)

Pigeons lack a gallbladder, although many birds, including chickens and gulls, have one. A comparative study of pigeon and gull livers found the gallbladder absent in every pigeon examined; bile leaves the two-lobed liver through two bile ducts that open into the duodenum. The Weber State dissection guide makes the same observation for the rock dove. (Khoder & Hameed (2025): liver and biliary system of the pigeon and gull, Iraqi Journal of Veterinary Sciences.)

For a student, the practical point is not to search for a missing organ: the liver's two hepatic ducts are the whole biliary route. Many textbook diagrams are drawn from the chicken and show a gallbladder, so check which species a diagram shows before you label a pigeon. (Cavitt, Weber State University: external and internal anatomy of the rock dove (PDF).)

A morphological study of domestic pigeons described paired cervical air sacs, a single clavicular sac formed by the fusion of two bilateral diverticula, cranial thoracic sacs whose diverticula extend into the second to seventh ribs, smaller cranial than caudal thoracic sacs, caudal thoracic sacs with no diverticula into bone, and abdominal sacs with cranial and caudal parts. That gives nine air sacs, the usual figure for birds. (Hussein, Ibrahim & Ali (2017): morphology of the air sacs in domestic pigeons.)

Some veterinary texts based on the chicken describe eight air sacs, because sacs joined across the midline are counted as one; poultry sources such as Poultry Hub count nine. The difference is a matter of counting, not of function: in every bird the sacs work as bellows, the lungs barely change volume, and there is no diaphragm. Air sac walls are only a few cells thick, which is why they appear as glistening, cobweb-like films when the body cavity is opened. (WikiVet: avian respiration, anatomy and physiology.)

In flying birds the pectoral muscles make up roughly 15–25% of body mass, and together with the supracoracoideus 25–40%. The pectoralis pulls the wing down and forward in the power stroke; the smaller supracoracoideus, lying under it against the keel, lifts and twists the wing through its tendon pulley at the shoulder. (Wikipedia: supracoracoideus and flight muscle mass.)

Measurements in pigeons show the size difference: Tobalske and Biewener recorded single-side muscle masses averaging about 53 g for the pectoralis and 10 g for the supracoracoideus. Hummingbirds, which hover and generate about a quarter of their weight support on the upstroke, have a proportionally larger supracoracoideus than other birds. (Tobalske & Biewener (2008): the pigeon supracoracoideus in flight, Journal of Experimental Biology.)