A. Size.
1. One of the smallest diurnal birds of prey is the pygmy falcon (Polihierax semitorquatus) of Africa, which reaches a weight of about 60 g (2.1 oz.) and has a wingspan of about 0.3 m (1 ft.) (Pickford and Tarboton, 1989). The smallest North American diurnal bird of prey is the American kestrel. American kestrels weigh about 120 g (4.2 oz.) and have a wingspan of about 0.2 m (0.7 ft.) (Johnsgard, 1990).
2. The largest diurnal bird of prey is the Andean condor, which weighs up to 14 kg (31 lb.) and has a wing span of up to 3.2 m (10.5 ft.). The largest North American bird of prey is the California condor with a wingspan of up to 2.9 m (9.5 ft.) (Martin, 1987). The tallest diurnal bird of prey is the secretary bird with a height of up to 1.3 m (4.3 ft.) (Pickford and Tarboton, 1989).
3. Most species, like Cooper's hawks, are sexually dimorphic (males and females are distinctly different in appearance). In diurnal birds of prey, males and females are usually similar in coloration, but the females are often larger than the males in size. In general, species feeding on carrion, like turkey vultures, show no sexual dimorphism.
B. Body shape.
1. In general, the bodies of diurnal birds of prey are fusiform in shape (rounded and tapering at both ends). This body shape reduces drag while flying.
C. Coloration.
1. Most diurnal birds of prey have subdued feather colors, mainly browns, rust, black, white, and some blues and grays.
2. Old and New World vultures may have subdued feather colors, but the skin color of their heads and/or necks can be quite vivid. The king vulture (Sarcoramphus papa) is a striking example, with a head and neck boldly patterned in yellow, orange, and black.
3. Males and females are usually identical in color except in some harriers and falcons. In merlins (Falco columbarius) and American kestrels, the males' wings are gray-blue from above while the females' are brown.
4. Many species have an immature plumage and an adult plumage. Immature bald eagles are completely brown in coloration, while adults have a distinctive white head and tail.
5. Some species, especially hawks in the genus Buteo, have light and dark color morphs (variations). This polymorphism seems to be related to habitat. In red-tailed hawks, dark colored birds are more common in the open country of the western U.S., while light colored birds are more prevalent in the forested areas of the eastern U.S.
D. Limbs.
1. Forelimb.
a. The forelimbs of birds are modified into wings.
b. Wings are covered with feathers, providing a lightweight surface for lifting and propelling a bird in flight.
(1) The primary flight feathers are the largest, outermost feathers of the wing, and they attach to the hand bones. The primaries (usually ten per wing) provide lift, but are mainly used for propulsion during flight.
(2) The secondary flight feathers attach to and run the length of the forearm. These feathers provide most of the lift in flight and can number up to 35 in larger birds, such as bateleur eagles (Brooke and Birkhead, 1991).
(3) The alula are three to four small feathers attached to the thumb bone. The alula helps increase lift by reducing air turbulence during flight.
(4) The covert feathers cover the base of the flight feathers on both the inner and outer surfaces of the wing (and tail).
c. Wing shape is an adaptation to a bird's foraging behavior.
(1) The majority of hawks and eagles have broad, rounded wings for high soaring on air currents and thermals. This type of wing doesn't produce great speed; therefore, these birds rely on surprise when catching prey.
(2) Falcons have relatively long, narrow, sharply angled wings for chasing or stooping (diving) on prey.
(3) Hawks of the genus Accipiter have relatively short, rounded wings for short bursts of speed and maneuverability while chasing prey in dense vegetation.
(4) Ospreys and harriers have long, rounded wings for stability when flying close to water or land, seeking prey.
d. The wings of many diurnal birds of prey have a vestigial claw located at the end of the thumb bone. Archaeopteryx had clawed fingers to help it climb trees; but, the wing claw found in Falconiformes is believed to be a nonfunctional, ancestral structure.
2. Hindlimb.
a. The hindlimbs are used for walking, running, jumping, perching, and grasping prey. In diurnal birds of prey, the legs are well- muscled and sturdy.
b. The legs and feet are covered with scales.
c. A bird's foot consists of four toes.
(1) In most diurnal birds of prey the first toe points backward and the remaining three point forward. In ospreys, the fourth toe can be moved backward, allowing two toes to face forward and two backward.
(2) The toes of some birds have specialized grasping surfaces. Fish-eating ospreys have spiny gripping studs on their toes for holding on to their slippery prey.
(3) Each toe has a claw, or talon. Talons are made of keratin, a tough, fibrous protein, and are usually curved downward.
(a) Most diurnal birds of prey have strong toes and sturdy, sharp talons for catching and carrying prey. Vultures, who feed primarily on carrion, have weak toes and talons.
(b) Birds that kill and carry large prey, like golden eagles (Aquila chrysaetos), have relatively long hind talons.
(c) Hind talons can also be used to strike down other birds in mid-flight, a technique often used by larger falcons, such as peregrines and gyrfalcons.
E. Head.
1. Eyes.
a. The eyes of diurnal birds of prey are set on either side of the head, facing forward.
b. The eye is protected by an upper and lower eyelid as well as a nictitating membrane.
(1) The nictitating membrane acts as a third eyelid, moving horizontally across the eye starting at the inner corner.
(2) A bird can close the clear membrane to protect the eye without losing vision.
(3) The membrane spreads fluid across the eye, keeping the eye moist. Birds most likely use the membrane when flying on windy days or when dust and debris are in the air.
c. Most hawks, eagles, and falcons have a distinctive shelf or "eyebrow" above and in front of the eye. The eyebrow is thought to shade the eye from the sun and provide some physical protection.
2. Nostrils.
a. Two nasal openings lie at the base of the upper bill, in the cere.
b. In some species, the nostrils have special modifications.
(1) An osprey has the ability to close its nostrils, which is an adaptation to diving into water after prey.
(2) Falcons have bony tubercles in their nostrils that may act like air baffles, slowing airflow through the nasal passages during high speed stoops.
3. Beak.
a. The bird's beak, or bill, is made of bone and covered by horny plates made of keratin.
b. The beak is strongly hooked at the tip and sharp on the edges. Modern birds don't have teeth.
c. The beaks of some species are modified according to feeding habits.
(1) In falcons, the upper mandible of the beak is notched, forming a "tooth." This tooth is used to break the cervical vertebrae of captured prey.
(2) The upper mandible of the snail kite is elongated for extracting snails from shells.
d. The base of the upper bill is soft and fleshy, forming the cere. Ceres are an adaptation to a meat-eating diet. A lack of feathers around the beak makes for easy cleaning after feeding.
4. Ears.
a. Diurnal birds of prey have two ear openings that lie just behind and below the eyes.
b. In most species the opening is covered by feathers, but it's visible on birds with unfeathered heads, like vultures.
c. Some harriers, like the northern harrier (Circus cyaneus), have owl-like facial disks that help funnel sound to their large ear openings.
F. Tail.
1. Tail feathers attach to the pygostyle or tail bone.
2. Like wings, tails create lift and balance in flight. Tails are also used as a rudder for turning, and a brake for landing.
3. Tail shape and length corresponds to a bird's hunting technique.
a. Birds dependent on fast, open flight to catch prey tend to have short tails (falcons).
b. Birds that need quick maneuverability pursuing prey through brush have long tails (Accipiter hawks).
c. The distinctive forked tail of the American swallow-tailed kite (Elanoides forficatus) provides extra lift as the bird flies at slow speeds capturing insects.
G. Skin.
1. Compared with other vertebrates, birds have thin skin.
2. Specialized cells in the skin produce feathers, scales, claws, and the outer layer of beaks. Like human fingernails and hair, these external structures are made of keratin.
3. Stored fat attached to the innermost layer of skin helps prevent heat loss as well as provide energy during migration.
4. Smooth muscles in the skin permit birds to fluff feathers (forming an insulating layer of trapped air) when the weather is cold, spread feathers when flying, or press feathers against the body (to lose heat) when the weather is warm.
5. Most birds have a uropygial, or preen gland, located on the upper surface of the rump and covered by feathers. The gland secretes oils used by the bird when preening. These oils are believed to help waterproof feathers and condition exposed skin. The oils may also have substances which control bacterial and fungal infections, as well as contain vitamin D.
6. Birds don't have sweat glands.
H. Feathers.
1. Feathers provide birds with insulation, skin protection, a streamlined surface for flight, and coloration.
2. There are six types of feathers: vaned, down, semiplume, filoplume, bristle, and powder down.
a. Vaned feathers are divided into contour feathers (on the body for streamlining and protection) and flight feathers (extending beyond the body for flight). Vaned feathers have a central shaft, creating two vane webs on either side. The webs are held together and flat by hundreds of barbs, barbules, and hooklets.
b. Down feathers are short, fluffy, insulating feathers, usually hidden under contour feathers.
c. Semiplume feathers are intermediate in structure between contour and down feathers. They're also found under contour feathers and are mainly for insulation.
d. Filoplume feathers have long whiplike shafts and are usually associated with the flight feathers. They're believed to have some type of sensory function.
e. Bristles are modified, vaneless contour feathers that commonly occur around the eyes, nostrils, or mouth. They're tactile or protective in nature.
f. Powder down feathers are the only feathers that grow continuously. They are scattered throughout the plumage and their tips crumble into a talclike powder. The powder, which is spread by the bird during preening, helps to waterproof and preserve the feathers.
3. Feathers aren't evenly distributed over the body, but grow along specific feather tracts.
4. Adult birds are completely covered with feathers expect for the legs, feet, beak, and sometimes head. Most vultures have featherless or sparsely feathered heads, which can be easily cleaned after feeding on carcasses.
5. Most adult birds molt (shed) and renew their feathers once a year after the breeding season. The old and worn feathers are actually pushed out of their follicles by the new feathers growing underneath.
6. In diurnal birds of prey, the feathers are molted in sequential patterns so at no time are the birds flightless, as in some species of waterfowl.
7. The speed of the molt depends on the species and location. In most species, the molt lasts four to eight weeks. Peregrine falcons in southern Europe molt in five to six weeks. Arctic peregrine populations molt for about two months in summer, stop the molt while migrating, then resume molting in their winter quarters. Large eagles may require two years for a complete molt (Welty, 1982).
8. Chicks are hatched with a natal down, which is replaced by a juvenile plumage before they leave the nest. The juvenile plumage may then be replaced by adult plumage or a number of immature plumages, depending upon the species. Bald eagles take four or five years to reach adult plumage (Scott, 1987).
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