Thorax, Abdomen, and Back
- Throat (also called the upper foreneck or gular region) extends from just under and below the lower mandible to the anterior margins of the sternum and breast musculature. This area is usually brightly coloured in many species, as are the lateral malar areas on either side of the throat.
Jugulum is the most ventral, midline region of the throat, and flanked by the sides of the throat.
Breast (also called the chest or pectoral region) is the area of feathering that extends over the musculature of the breast and covers about three-quarters of the breast muscle surface.
Abdomen (belly) is the most ventral area along the midline of the body. The region extends from the posterior one-quarter of the sternum to the vent, or cloacal area. In flying birds, this area is so long that the region actually overlies part of the posterior portion of the sternum and breast muscles.
Crural feathers cover the tibial portion of the leg. They blend in with the feathers of the abdomen. In Arctic birds, like the snowy owl (Nyctea scandiaca), the crural feathers extend distally to the tips of the birds toes.
Side (flank) is the lateral portion of the birds trunk and extends from the abdominal region up to the base of the wings. The sides are contiguous with the axillary area at the base of the wing and lie entirely under the wing when it is extended during flight.
Axillary region is the base of the ventral wing extending from the side out onto the ventral wing lining. This armpit area of the bird is distinctly marked in such species as the Black-bellied Plover (Pluvialis squatarola).
Flank is the lateral area posterior to the side region. Lying below the pelvis, it then extends back to the base of the tail.
Crissum (also called the undertail coverts, or circumcloacal region) is the area of loose feathers that surround the cloaca and includes the undertail coverts that cover the ventral base of the tail. In some species, this area can be distinctly colored, as with the grey catbird (Dumetella carolinensis).
Anal pteryla or cloacal circlet is the two rows of feathers that are arranged in concentric circles around the cloaca. They are mostly covered by the feathers.
- Nape (also called the upper hindneck or nuchal region) is the dorsal surface of the nect, from the occipital region of the skull posterior to the base of the neck and beginning of the thoracic vertebrae. The nape is often a dark colour that contrasts with the lighter colours of the ventral neck.
Back (also called the interscapular region or dorsum) is the dorsal region of the thorax, roughly the area between the wings. This area is laterally bounded by the scapular feathers which cover the dorsal bases of the wings.
Scapulars (also called the humeral region) are the feathers that overlie the scapula bone at the base of the dorsal wing. When a bird is perched or standing, the scapulars often cover much of the folded wing.
Rump (also called the uropygium, uropygial region, or lower back) is the area that overlies the pelvic bones (synsacrum and ilia). The lateral boundaries are the flanks along the side of the body just anterior to the tail. The rump is sometimes coloured in contrast to the back, as in the northern flicker (Colaptes auratus) and northern harrier (Circus cyaneus).
Upper tail coverts are the single row of feathers that cover the bases of the tail feathers (rectrices).
- Humerus is the short, thick bone at the base of the wing. As in reptiles and mammals, the avian humerus supports the other bones of the pectoral limb.
Radius and ulna are the supporting bones of the forearm portion of the wing. The ulna is the larger bone. The attachment of the secondary quills takes place to the posterior surface of the ulna.
Carpometacarpus is a fusion of the carpal (wrist) and the metacarpal (hand) bones. In birds, the distal wing skeleton is a much reduced and simplified version of the standard vertebrate forelimb plan. The carpometacarpus supports most of the primary remiges (the main flight feathers)
Digits: The tiny first digit is called the alula, and is located at the bend of the wrist area of the wing. The first digit phalanges (finger bones) support three small flight feathers, collectively referred to as the alula. Bony remnants of the second and third digits form the most distal portion of the wing skeleton.
Patagium, patagialis longus muscle and tendon, is the tough, fibrous sheet of tissue that connects the shoulder area of the wing to the carpal bones at the wrist. The patagialis longus actually forms much of the leading edge of the wing. The fleshier humeral patagium connects the elbow to the thorax. The only bones at the leading edge of the wing are the carpal bones and carpometacarpus at the bend of the wrist.
Postpatagium is the tough band of tendinous tissue that envelops and supports the quills of all the wing remiges, from elbow to wingtip. The postpatagium provides much of the elastic strength of the wing and keeps the flight feathers properly aligned and firmly attached to the wing skeleton.
Primary and secondary remiges are the two groups of flight feathers that relate to the bones and tendons of the wing. The primaries attach only to the bones of the manus (hand) while the secondaries attach only to the posterior surface of the ulna.
Dorsal Wing Surface
- Primaries (also called primary remiges singular, remex) are the major flight feathers of the wing (remiges). They are attached to the manus of the forelimb. The primaries form the main propulsive area of the outer wing, providing most of the worward thrust in active flight. There is a strong asymmetry of the primaries with each remex acting as an individual airfoil. Most birds have ten primary remiges, though many songbirds have just nine.
- Marginal (also called wing coverts) cover a significant portion of the anterior dorsal surface of the wing in larger birds. In smaller birds, the marginal coverts may be reduced to two to four rows of tiny feathers overlying the leading edge of the patagium. The marginal coverts are often more hairlike with more flexible vanes than those of the greater, median, and lesser coverts.
- Greater coverts of the primaries and secondaries consist of a single row of relatively large feathers which are adjacent to the primary and secondary flight feathers. These coverts shield the bases of the remiges. In most species, each remex is matched by one greater covert feather, although the greater covert of the most distal primary is very small and may appear to be missing in many groups.
- Median coverts of the primaries and secondaries consist of a single row of coverts just anterior or proximal to the greater coverts. In smaller birds, the secondary coverts may be difficult to distinguish from the marginal coverts.
- Lesser coverts of the primaries and secondaries consist of the last two or three distinct rows of covert feathers between the larger coverts and the small, soft feathers of the marginal coverts.
Secondaries (also called secondary remiges) are attached to the ulna of the forearm. Each secondary remex is connected to the trailing edge of the ulna. The secondaries form the trailing edge of the wings airfoil. In larger soaring birds, the secondaries make up most of the surface area of the wing. The number of secondaries varies with wing length in a species and range from a low of nine in most songbirds to twenty-five in the larger vultures.
Tertiaries (also called tertial or humeral feathers) are the three or four feathers proximal to the innermost secondaries. True tertiaries are not arranged in the same row as the secondaries but the innermost three or four secondaries of some species are sometimes mistakenly called tertiaries.
Scapulars (also called the humeral region) are the feathers that overlie the scapula at the base of the dorsal wing.
Alular Quills are collectively called the Alula. Usually, they consist of three small, stiff quills that arise from the first digit of the manus. The alula acts as an aerodynamic slot and spoiler, aiding and disrupting air flow over the wing in flight. Alular quill coverts consist of a separate area in larger birds that protect the base of the alular quills and may be distinguished from other marginal coverts.
Ventral Wing Surface
- Primaries are the major flight feathers of the wing. Attached to the manus, they form the main propulsive area of the outer wing and provide most of the forward thrust in active flight. There is a strong asymmetry of the primary remiges with each remex acting as an individual airfoil. Most species have ten primary remiges, though many songbirds have just nine.
Secondaries are the feathers (remiges) attached to the ulna of the forearm. Each secondary remex is attached to the trailing edge of the ulna. The secondaries form the trailing edge of the wings airfoil. In larger soaring birds, the secondaries make up most of the surface area of the wing. The number of secondary remiges in a species varies with wing length, ranging from a low of nine in many songbirds to twenty-five in larger vultures.
Tertiaries are the three or four feathers just proximal to the innermost secondaries. True tertiaries are not arranged in the same row as the secondaries, but the innermost three or four secondaries of some species are sometimes mistakenly called tertiaries.
Greater coverts of the primaries and secondaries consist of a single row of relatively large covert feathers adjacent to the primary and secondary flight feathers. These coverts shield the bases of the remiges. In most species, each remex is matched by one greater covert feather, although the greater covert of the most distal primary is very small and may appear to be missing in some groups.
Wing lining or marginal coverts are very soft feathers that form a smooth, featureless surface on the anterior edge of the ventral wing. Although the wing lining is made up of rows of coverts, in most species the individual rows of coverts cannot be readily distinguished.
Axillaries or axillar region consists of the relatively long and stiff covert feathers that cover the ventral base of the wing; that is, the armpit area. In most species, these feathers are white, although, in a few, as the black-bellied plover (Pluvialis squatarola) and the prairie falcon (Falco mexicanus), the axillaries are distinctly dark and are a good field mark.
- Forehead or frontal region extends from the base of the beak posterior to a line drawn between the anterior angle (nasal canthus) of each eye. The ventral margin is a line drawn between the nostril and the nasal canthus of the eye.
Crown or crest is the region just posterior to the forehead. The crown extends from the forehead back to the beginning of the cervical vertebrae of the neck. In crested birds the crown feathers are lengthened.
Pileum of the head is the entire top of the head and includes the forehead, crown, and occipital regions.
Lore or loral region (plural, lores) is the lateral area of the head just posterior to the bill and anterior to the nasal canthus of the eye. The commissure of the mouth forms its lower border. In some birds, it is fleshy and naked of feathers and often brightly colored, especially during breeding season.
Superciliary line or supercilium is a strip of contrasting feathers located immediately above the eye and along the side of the head. It is found in many birds, but mostly in sparrows.
Eye ring or rimal feathering consists of two to four tiny concentric bands of feathers surrounding the eyes at the edge of the eyelids. Often colored in contrast to the surrounding plummage, it forms a distinctive eye ring like that in Swainsons thrush (Catharus ustulatus). It is absent in the pigeon family (Columbidae).
Eyeline is a line that extends from the posterior angle of the eye (temporal canthus) and runs posterior toward the nape of the neck. It is often an important field mark.
Narial feathers are long feathers at the base of the maxilla (upper bill), extending anteriorly to partially cover the nostrils. It is present in such groups as crows (Family Corvidae).
Rictal bristles are bristlelike feathers around the corner of the mouth. They are present and well-developed in many passerines (songbirds), as well as in other groups that feed on flying insects. They may serve a tactile, sensory function, particularly in nocturnal species, where they act like a cats whiskers. They are absent in the pigeon family (Columbidae)
Auricular feathers (also called auriculars or ear covers) consist of a wide lateral patch of feathers just ventral and posterior to the eye and cover the ear opening. Auriculars are arranged in concentric bands that extend downward and back from the eye, covering the lateral cheek area of the head below the eye.
Malar region (also called malars or mustache feathers) is a patch that extends posteriorly and ventrally from the mandibular famus ventral to the commissure of the mouth and inbetween the auricular feathers of the cheek and the throat feathers. This area can be boldly marked as in the male northern flicker (Colaptes auratus) or the prairie falcon (Falco mexicanus).
Commissure (also called the commisural point or sometimes incorrectly, the gape) is the angle at which the maxilla and the mandible meet. Gape more properly refers to the gap between the open maxilla and the mandible when the bill is opened wide.
Nape region (also called the upper hindneck or nuchal region) is located on the dorsal surface of the neck from the occipital ridge of the skull or first cervical vertebrae posterior to the base of the neck.
Upper mandible or maxilla is the combined upper jaw and bill.
Lower mandible is, technically, the lower jaw or the lower jaw and lower bill. In ornithological literature, however, the mandible may be either the upper or the lower bill.
Mandibular tomium (plural, mandibular tomia) is the cutting edge of either the upper or the lower mandible.
Culmen is the central midline ridge running from the tip of the upper bill back to the base of the bill.
Gonys is the central midline ridge that runs from the tip of the upper bill back to the base of the bill.
Mandibular rami (singular, ramus) consist of two lateral halves of the lower jaw between the anterior synthesis and the quadrate articulation.
Rhamphotheca is the horny sheath that covers the bill.
Nasal and temporal canthi consist of the angles of the eye where the upper and lower eyelids meet. The nasal canthus is the anterior angle of the eye. The temporal canthus is the posterior corner of the eye.
Operculum is a soft, fleshy structure located at the base of the bill and covers the external naris (nostril) in such birds as pigeons and starlings.
Today, the terms bill and beak are used interchangeably. Originally, beak referred to the sharp, decurved bills of birds of prey like hawks and falcons.
Birds depend on their bills to:
- obtain food
- preen their feathers
- build their nests
- perform courtship displays
- defend themselves
The bill is composed of a bony framework covered by a tough jacket of keratin that forms the distinctive shape.
The upper bill, which is slightly mobile, is supported by the maxilla and other bones of the skull. The pterygoid, quadrate, and zygomatic arch bones that support the maxilla can slide forward or backward. At the forehead, the maxilla bones join the skull at a thin, flexible sheet of nasal bones (nasofrontal hinge). In most bird species, the upper bill is much more flexible than it may appear.
Examples of Bill Types
- White-throated Sparrow (Zonitrichia albicollis) has a light bill that is adapted to small seeds and plant material.
- Evening Grosbeak (Hesperiphona vespertina) has a heavy bill adapted for seed-cracking.
- Atlantic Puffin (Fratercula arctica) has a tall, thin beak that is adapted to catching and holding small fish.
- Golden-winged Warbler (Vermivora chrysoptera) has a probing beak designed to feed on small insects.
- Whimbrel (Numenius phaeopus) has a long bill adapted to probing mudflats, looking for small invertebrates.
- Pileated Woodpecker (Dryocopus pileatus) has a long, heavy bill used for chopping away wood to expose insects.
- Keel-billed Toucan (Ramphastos sulfuratus) has a bill-shape that makes it easier for it to feed on fruits from rain-forest trees.
- Great Blue Heron (Ardea herodias) has a long bill adapted for stabbing and seizing small animals and fish.
- Greater Flamingo (Phoenicopterus ruber) has a unique bill that has a complex filtering system to strain small invertebrates from shallow mud flats and lakes.
- Sword-billed Hummingbird (Ensifera ensifera) has an extremely long bill designed for probing deep into the bells of tubular tropical flowers.
- Red-tailed Hawk (Buteo jamaicensis) has a sharp, decurved bill that is adapted to tearing flesh from small animals.
Songbirds have the most delicate bills, designed to feed on small insects.
Omnivorous birds, such as thrushes and jays, have sturdy, generalized bills suited to a diet of plants or animal foods.
Grosbeaks and parrots have huge conical bills adapted for crushing hard nut shells.
Raptors and vultures generally have decurved bills with sharp cutting edges for dismembering prey.
Mergansers are fish-eaters and require long, thin bills with distinctive serrated edges that help hold onto slippery fish.
Feet of birds consist of different types of toe arrangments: Anisodactyl, Heterodactyl, Pamprodactyl, Syndactyl, and Zygodactyl.
Types of feet include: Booted, Lobate, Palmate, Raptorial, Reticulate, Scutellate, Scutellate-booted, Scutellate-reticulate, Semipalmate, and Totipalmate.
Anisodactyl describes a foot having three toes in front and one behind. It is the most common arrangement of the avian toe and the one seen in songbirds and perching birds.
Heterodactyl foot is similar to the Zygodactyl foot except that the inner (second) toe is reversed in the heterodactyl type. This is to help the short, weak first digit in gripping branches. This type is found only in trogons (Order Trogoniformes, Family Trogonidae).
Pamprodactyl foot has all four toes in front, as seen in most swifts (Family Apodidae). The first and fourth digits pivot freely foreward and backward. Swifts often rotate all four toes forward and use their tiny feet as hooks to hang while roosting on the walls of chimneys, caves, or hollow trees.
Syndactyl foot has two front toes (second and third digits) partially joined or webbed for much of their length. This foot pattern is common in kingfishers (Family Alcedinidae), hornbills (Family Bucerotidae), and rollers (Order Coraciiformes).
Zygodactyl foot has two toes facing forward and two facing backward. This is the second most common toe arrangement in perching birds. It is found in the osprey (Family Pandionidae) , most woodpeckers (Family Picidae), owls (Order Strigiformes), cuckoos, parrots, mousebirds, and some swifts.
Types of Feet
Booted feet occur when the tarsus is covered by several long, continuous plate-like scales, with no small overlapping scales. Booted feet are found in the thrushes (Family Muscicapidae)
Lobate feet have toes or lobes. The anterior digits (2, 3, and 4) are edged with lobes of skin that expand or contract as the bird swims. Lobate feet are found in grebes, though some palmate-footed ducks have lobes of skin on the hallux.
Palmate means that 3 toes are completely webbed. However, only the anterior digits (2, 3, and 4) are connected while the first digit is separate and posterior. The is the most common type of webbed foot and found in ducks, geese, swans, gulls, terns, and other aquatic birds.
Raptorial refers to birds of prey that have feet with long, strong toes and long, sharp, curved claws. These heavy claws are for catching, holding, and killing prey and found in kites, hawks, eagles, and falcons.
Reticulate means forming a network. This type occurs where the tarsus is covered not by a row of overlapping scales but by a fine patchwork of small, irregularly shaped plates in a reticulated, or netlike, pattern. Reticulate feet are found in many birds, including falcons and plovers.
Scutellate means covered with bony plates or scales, found in most birds with bare (unfeathered) legs and common in songbirds. In the scutellate foot, the tarsus and foot are covered with a tough layer of horny keratin scales called the investment. In most birds, as the blue jay (Cyanocitta cristata), the scales are arranged in an overlapping (imbricated) row along the anterior edge of the tarsus and foot.
Scutellate-booted feet occur when the tarsus is scutellate along the anterior edge but is covered behind by a single, long, booted scale. This type of foot is found in such songbirds as the grey catbird (Dumetella carolinensis).
Scutellate-reticulate feet occur when the tarsus and foot are covered in a combination of reticulated and scutellate scale patterns. For example, the anterior tarsus of the rock dove (Columba livia) is covered with a row of imbricated scutes; but the posterior of the tarsus is covered with reticulated scales.
Semipalmate is partial webbing that occurs only at the base of the anterior toes (2, 3, and 4). This type is found in some sandpipers and plovers, all grouse, and some domesticated breeds of chickens.
Totipalmate refers to all 4 toes being connected by webbing. This type is found in such aquatic birds as gannets, boobies, cormorants, and pelicans.
This page was updated in June 2006.