A. Skeletal system.
1. The skeleton gives a bird protection, support, and movement. It's also a site for calcium storage and the production of red blood cells.
2. A bird's skeleton is made of bone. Many of the bones are fused or modified for greater strength and rigidity in walking, jumping, running, perching, and especially flying.
a. The sternum, or breastbone, is more developed than in other vertebrates. It has a large flattened plate, called a keel, for the attachment of flight muscles.
b. The wing attaches to the body via a sturdy pectoral girdle. In birds the pectoral girdle is made of three bones, rather than two as in humans. The furcula or "wish bone" is part of the pectoral girdle.
c. The wrist bones of the wing are fused and elongated and only three finger bones (digits) remain.
d. The pelvis is long and has a forward opening to facilitate egg laying.
e. The thigh bones are generally shortened and sturdy. The foot and ankle bones are fused and elongated.
f. The thigh is held close to the body and covered by feathers. The visible backward bending leg joint corresponds to the ankle joint in humans. Birds stand on their toes.
g. Several vertebrae are fused except for those in the neck region. The number of neck vertebrae for birds in general varies from 15 or fewer in small birds with short necks to 20 or more in large birds with long necks. More neck vertebrae means increased mobility of the head and neck. California condors have 18 neck vertebrae (Mundy, et al., 1992).
3. The bird's large eyes are separately supported and protected by an encircling ring of small, shinglelike bony plates called the sclerotic ring.
4. Many of the bones, including those in the skull, are pneumatized (filled with air). Light, air-filled bones are an adaptation for flight. A bird's skeleton comprises only about 5% of its total body weight (Brooke and Birkhead, 1991).
B. Musculature system.
1. Flight muscles.
a. In diurnal birds of prey most of the muscle mass is divided between the flight and leg muscles. The major muscles are situated close to the bird's center of gravity for proper balance.
b. The largest flight muscles are located in the breast area and attach to the keel. The massive pectoralis muscle pushes the wing downward in flight and supports the bird while soaring. The opposing supracoracoideus muscle raises the wing in flight.
c. The flight muscles are red, not white, as in chickens or turkeys. The coloration is due to the abundance of oxygen-carrying myoglobin, typical of birds that are strong, long-distance flyers.
2. Leg muscles.
a. Diurnal birds of prey have strong leg muscles for grasping and carrying prey with their toes. They're one of the few bird groups that have well developed muscles running the length of the leg. b. The toes and talons are controlled by a complex series of tendons attached to muscles in the upper part of the leg.
3. Jaw and neck muscles.
a. The jaw and neck muscles are substantial and used for tearing and eating flesh.
4. Tail muscles.
a. Complex rump muscles raise, lower, and spread the tail during flight, take offs, landings, and courtship.
C. Nervous system.
1. The nervous system of birds is divided into two basic parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves and ganglia).
2. Important and relatively large structures of the brain include the cerebral hemispheres, optic lobes, and cerebellum.
a. The cerebral hemispheres (cerebrum) are the dominant coordinating centers of the brain. The cerebrum controls most of the body's activities as well as instinctive and conditioned behavior.
b. The optic lobes receive and process sensory information from the eyes. The lobes control eye and neck muscles, enabling birds to quickly track moving objects or avoid danger.
c. The cerebellum controls the bird's posture and balance.
3. The spinal cord extends the length of the vertebral column with bundles of both motor and sensory nerves.
D. Circulatory system.
1. The circulatory system is similar to those in other vertebrates. As in mammals, birds have a four-chambered heart; however, a bird's heart is proportionately larger and more powerful.
2. Birds usually have higher metabolic rates than mammals and need larger, more efficient hearts.
a. Birds require large amounts of energy for flight, and need efficient oxygen circulation in high altitudes. The highest flight recorded for a bird was 11,274 m (37,000 ft.) when a Ruppell's griffon vulture collided into a commercial airline over western Africa (Martin, 1987).
3. Birds normally maintain a body temperature of 380C to 420C (100.40F-107.60F) (Brooke and Birkhead, 1991). They thermoregulate in a variety of ways.
a. Arteries and veins in the head and legs of many birds form heat exchangers called retia mirabilia. A net of vein and arterial vessels lie side by side, allowing outgoing arterial blood to pass heat to the incoming venous blood. This cools the outgoing blood and warms the incoming blood, minimizing heat loss in areas with little or no feathers.
b. Birds also stay warm by increasing their activity rate (metabolism), fluffing feathers to trap insulating air, shivering, or tucking exposed parts, such as faces and legs into feathers.
c. To stay cool, birds can decrease their activity rate, sleek feathers flat to get rid of trapped air, or pant. New World vultures have an interesting cooling method known as urohidrosis. They squirt liquid excrement on their legs, which are cooled as the water in the excrement evaporates.
E. Digestive system.
1. The esophagus is generally wide and acts as a food storage area when large prey items are swallowed. Diurnal birds of prey have an additional storage chamber (a saclike swelling of the esophagus) called the crop.
2. The stomach has two chambers.
a. The first chamber is the glandular stomach, or proventriculus, which produces gastric juices for chemical digestion.
b. The second chamber is the muscular stomach, or gizzard, which mechanically digests food items.
c. Food moves back and forth between the two chambers until properly digested.
d. The gizzard in most diurnal birds of prey is relatively thin-walled and saclike due to the soft nature of fish and meat. Insect-eating birds have more heavily muscled gizzards, which may contain substantial quantities of grit, pebbles, or sand that aid in the breakdown of hard foods.
e. The gizzard traps indigestible items such as bones, feathers, fur, and teeth. These items are rolled into compact "pellets" and then regurgitated.
3. The intestine is arranged in a number of U-shaped loops. Digestion is completed in the intestine, and nutrients are absorbed through the gut wall into the bloodstream. In general, meat-eating birds have shorter intestinal tracts than grain or fish-eating birds.
4. Both the liver and pancreas are relatively larger than in mammals, but
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