In the description and identification of birds, reference is often made to certain external features and regions of the body. To make the best use of these descriptions, a familiarity with the topography of birds is essential. The basic regions of the body are the head, neck, bill, trunk, wings, legs and feet, and tail. We will examine the external features associated with these regions in this order.
THE HEAD
The head is divided into several regions; often the coloration of these regions is an essential identifying characteristic of a species. Anterior to the eyes and above the beak is the forehead. On top of the head is the crown, and posterior to the crown is the occiput. Just below the forehead and crown and above the eyes is the superciliaryline. The crown and superciliary line are distinctively colored in some birds, especially among the fringillid species. In some species there is an area of thickened bare skin just above the upper mandible and adjacent to the forehead called the cere. It may be brightly colored in species such as parrots and birds of prey. The nostrils, or externalnares, are situated near the base of the upper mandible or in the cere in most birds. However, there are some interesting exceptions. The kiwi, which is renown for its olfactory sense, has its nostrils located at the tip of the bill. In the procellariiformes the nostrils are extended forward in a covered tube, hence the name "tube-nosed" birds. In pigeons, a swollen area of tissue, called the operculum surrounds and caps each nostril.
On the side of the head, the area around the eyes is called the orbital region and consists of the eye itself, the eyelids, and the eye-ring (feathers immediately around the eye which are distinctively colored in some birds). The corner of the eye closest to the nasal area is called the nasal or medial canthus; the opposite corner is called the temporal or lateralcanthus. The area between the eye-ring and the bill may also be distinctively colored and is called the lore (plural, lores).
Eye color is a product of the pigmentation of the iris, the circular smooth muscle which regulates the amount of light entering the eye. It is often a distinguishing character not only between species, but between age groups. For example, eye color in juvenile buteo hawks is a pale amber, while that of adults is a darker brown. The eyelids close the eye in birds, just as in mammals, except that it is accomplished by raising of the lower lid, rather than lowering of the upper lid. On the nasal side of each eye between the cornea and the eyelid is a translucent membrane that is pulled obliquely across the eye for protection, called the nictitatingmembrane, or nictitans. In most birds, the blink reflex involves the closing of the nictitating membrane only, and the lids (and nictitating membrane) close only during sleep.
Posterior to the orbital region on the side of the head is the auricular region, the area surrounding the ear opening. The feathers in this area are called auriculars; they may completely conceal the ear opening. Birds do not have fleshy ear lobes or external pinna as mammals do, but there is a slight thickening of the skin into a fold around the ear. The earopening communicates with the middle ear via a passage called the externalacousticmeatus. The ear opening itself may be very different in size in different species, and may in fact have an asymmetrical shape and location in owls, which aids their auditory discrimination.
The area of the head which would correspond to your cheek, that is from the base of the lower bill to the angle of the jaw, is called the malar region. Its boundaries are the lore, the orbital region, the auricular region, and the lower jaw.
On the ventral side of the head, the sparsely feathered area between the rami of the lower jaw is the chin. The gular region posterior to the chin, continues to an imaginary line between the angles of the jaw.
THE BILL
The bill consists only of the upper and lower mandibles,
which lie on either side of the mouth. The mandibles are bony extensions
of the skull covered by thickened and horny epidermis that forms a sheath
of keratin (rhamphotheca) around the bony core. The cutting edges
of the mandibles are called tomia (singular, tomium), but
in some species, the bill may be hardened and/or sharp only at the tip
while the sides remain relatively soft and blunt (such as in ducks). The
tomium of each mandible consists of the hard cutting edge, the tomiumproper,
and a softer, fleshy part behind the angle of the mouth called the rictus.
THE NECK
The neck extends from the posterior margin of the occiput in the head to the anterior margin of the trunk at the thoracic inlet. The number of cervical vertebrae is highly variable in birds, ranging from 8 to 24. Consequently the neck length is also highly variable. The dorsal part of the neck is referred to as the nape, and the ventral part is the jugulum. The "throat" of the bird is really comprised of both the gular region of the head and the jugulum of the neck. The sides of the neck naturally run between the nape and jugulum and extend from the posterior borders of the auricular and malar regions of the head to the anterior region of the trunk.
THE TRUNK
The trunk comprises the bulk of the bird's body. Note its fusiform shape, in the lateral aspect. This is an essential feature aiding aerodynamic efficiency. Manipulate the trunk with your fingers and ascertain that it is a rigid, inflexible structure. Dorsally, it consists of the back and the rump. The back is arbitrarily defined as the anterior two-thirds of the area between the nape and the base of the tail; the rump is the posterior one-third of that area. On the dorsal surface of the posterior end of the trunk is a prominent papilla containing the opening of the oil or uropygial gland, from which the bird obtains oil for preening its feathers.
Ventrally, the trunk consists of the breast and the abdomen. Run your fingers along the mid-ventral surface and feel the ridge of the sternum and the prominent pectoral muscles on either side of the sternum. The abdomen, the area posterior to the sternum, is much more malleable (compressible) and terminates in the vent, or cloacal aperture. This transverse opening has protruding lips and is surrounded by a ring of short, stiff feathers called the anal circlet. The sides of the trunk are the areas under the wing, lateral to the breast, overlying the ribs. The area between the abdomen and the rump along the side of the body is referred to as the flanks.
THE WINGS
The wings are modified pectoral appendages arising from the shoulder
girdle, specifically adapted for flight. The bird wing has the same skeletal
plan and uses most of the same terminology as the human arm, although there
is extensive modification of the bone structure. In the partially extended
bird wing there are two prominent angles: the one at the wrist between
the hand and forearm (or antebrachium), the one at
the elbow, between the forearm and the upper arm, or brachium.
The wrist angle points forward, the elbow angle points backward, giving
the wing the shape of a "Z" fallen over on its top.
FIGURE 2. Lateral view of the head and body.
Label the significant areas of the neck and trunk.
Label the divisions of wing coverts, primaries, and secondaries.
The feathers of the wings belong to several functional groups. The flight feathers or remiges (singular, remex) arise from the arm and hand (or manus), and are long stiff quills. The primary remiges arise from the hand and wrist. They are often asymmetrical feathers, especially toward the distal tip of the wing, whereas the secondary remiges arise from the forearm and are symmetrical quills. In some long-winged species, flight feathers, called tertiaries, arise from the humerus.
The feathers that cover the bases of the remiges and project out over
the quills are the coverts: underwing coverts, located ventrally,
and overwing coverts, located dorsally. The coverts provide the flight
feathers with a more streamlined and aerodynamic profile as they slice
through the airstream. These major groups are subdivided into primary and
secondary coverts which cover the primary and secondary flight feathers,
respectively. They are again subdivided by rows: those that directly overlap
the bases of the primaries are the greater primary coverts; the
next row up is the median primary coverts; the next row up is the
lesser
primary coverts. The secondary coverts are subdivided similarly into
the greater, median, and lesser. Above the row of lesser coverts
is a row of marginal coverts. The underwing coverts are similarly
subdivided, although the row arrangement is less distinct. In some cases
only the greater primary and secondary (underwing) coverts are easily
differentiated.
Shade in or color and label the major divisions of the wing coverts, primaries,
secondaries, scapulars, axillars, patagium. Number the primary and secondary
feathers.
One to several shortened, stiffened feathers project from the thumb
and are called the allula. The purpose of these few feathers is
to open a slot for air to be forced down over the top of the wing. A group
of feathers arising from the shoulder and proximal part of the upper arm
are the scapulars.
The axillars are the feathers lying under
the wing and very close to the body. They are generally longer than the
underwing coverts and may be highly modified in some species. A conspicuous
feature of the extended wing is the patagium, the fold of skin which
extends from the upper arm along the entire lower arm to the wrist. There
is also a smaller fold of skin extending from the upper arm to the trunk,
called the humeral patagium. The patagium is essential because it
presents a flattened plane to the airstream, and also increases the surface
area of lift in the wing. A more detailed examination of the feathers and
bones of the wing and their relationship to flight mechanics will be presented
in Lab No. 3.
One to several shortened, stiffened feathers project from the thumb and are called the alula. The purpose of these few feathers is to open a slot for air to be forced down over the top of the wing. A group of feathers arising from the shoulder and proximal part of the upper arm are the scapulars. The axillars are the feathers lying under the wing and very close to the body. They are generally longer than the underwing covers and may be highly modified in some species. A conspicuous feature of the extended wing is the patagium, the fold of skin which extends from the upper arm along the entire lower arm to the wrist. There is also a smaller fold of skin extending from the upper arm to the trunk, called the humeral patagium. The patagium is essential because it presents a flattened plane to the airstream, and also increases the surface area of lift in the wing. A more detailed examination of the feathers and bones of the wing and their relationship to flight mechanics will be presented in Lab No. 3.
THE LEGS AND FEET
The legs and feet of the bird also follow the human structural plan, with some slight modification. The major regions of the leg are the thigh (that segment immediately adjacent to the abdomen), the knee joint between the thigh and lower leg, the lower leg, sometimes referred to as the shank or "drumstick", or more scientifically the crus or tibiotarsus, the heel or hock joint, and the foot, which is composed of the tarsometatarsus and the toes, or phalanges (singular, phalanx).
The knee joint is often close to the body and not easily observed in live birds, but it points forward, just as it does in mammals. The heel joint points backward, the two together giving the bird leg the shape of a backward "Z". The legs of most birds are feathered down to the knee joint, leaving the crus and feet bare. These latter structures are covered by scales or scutes, overlapping plaque-like sheets of horny epithelium. The free ends of the scales are directed distally and serve as protective barriers for underlying tissues. Several types of tarsal scutellation have been described in passerines and have been used extensively in taxonomy. In some species, such as grouse, sandgrouse, and many owls, the crus and tarsus are wholly or partially feathered.
There are four toes extending from the fused tarsometatarsal bones in the foot. Generally, three toes extend anteriorly and one toe points posteriorly (the hallux), although this is variable between species. The toes terminate in a claw, a pointed horny cap over the distal phalanx. The claws are specialized scales which are adapted for digging, scratching, clinging, even fighting. Often the claw is curved because the upper surface tends to grow faster than the lower one. The claw is constantly being renewed from its base, as it wears down at the tip. The bottom of the foot is also protected by scaly mounds, called metatarsal or digital pads. These are rounded elevations of the skin and subcutaneous fascia located primarily at the joints in the foot, and covered with a thin, scaly epidermis. In birds which use their feet for grasping prey, the pads are very rough, with increased surface area for clinging to their food.
THE TAIL (Locate the following structues on Fig. 2.)
Like the wings, the tail is composed of quills or rectrices (singular, rectrix) and tail coverts. The rectrices are paired on each side of the midline, and are of variable length, giving the tail its characteristic shape for that species. The undertail coverts are separated from those of the abdomen by the vent. They are collectively referred to as the crissum. The uppertail coverts are less clearly demarcated from the rump. The tail is supported by a bony and muscular structure called the pygostyle, more commonly known as the "pope’s nose".
FIG. 4. Leg and foot of the bird.
Label the important structures of the leg and foot.
Picture #4
MAKING MEASUREMENTS OF BIRDS
Measurements of certain topographical features of birds may be done for a number of reasons. Their primary use is in systematics, to aid in describing differences or similarities between species, genera, or other taxa, or to document the extent of variability within the taxa. Measurements of specific morphological characteristics are an aid to the determination of sex identification during the non-breeding season. Morphological measurements are also made to document growth of young. The taking of measurements on birds has been standardized so that data from different sites, taken by different investigators, on different occasions can all be combined for one taxa.
Procedure:
1. Measure the following morphological features on several study skins of a particular species and record the data in the table below. Refer to Fig. 5. Use dividers to measure the distance between points from the study skin, then line the divider points up on a ruler to measure the actual distance in cm or mm units.
