|MadSci Network: Zoology|
A - Let me answer this from two slightly different avenues - Simple and Complex: (Make sure to read all of this if you want to know a lot about birds!) Simple - Even though birds have relatively hollow bones (with large air spaces) they still have bone marrow cells, where red blood cells are produced. The marrow cavity is just smaller. Also, because birds have more efficient lungs than mammals, this works just fine for them. Complex - The answer is that although a bird's bones are filled with air, there is still a good deal of bone tissue left in the bones, enough to produce the red blood cells. Neat trivia (if you don't read the attached blurb): A bird's bones are connected to the air sacs, so if a bird breaks a large enough bone, it can actually breathe through it. Super Complex - Skeletal system. 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). The skeleton gives a bird protection, support, and movement. It's also a site for calcium storage and the production of red blood cells. 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. 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. 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. The wrist bones of the wing are fused and elongated and only three finger bones (digits) remain. The pelvis is long and has a forward opening to facilitate egg laying. The thigh bones are generally shortened and sturdy. The foot and ankle bones are fused and elongated. 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. 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, 1992). The bird's large eyes are separately supported and protected by an encircling ring of small, shinglelike bony plates called the sclerotic ring. A bird's 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. This means that less oxygen has to be supplied to the muscles. A lower red blood count would not hinder a bird. 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 to help facilitate the spreading of oxygen faster and more efficiently. Birds usually have higher metabolic rates than mammals and need larger, more efficient hearts. 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). Air enters the lungs from the nasal chambers and through the trachea. Unlike mammals, birds don't have a diaphragm. Instead, air moves into the lungs by a complex system of nine air sacs and interconnecting tubes. The respiratory system behaves like a two-cycle pump. (1) During the first inspiration, air passes almost completely into the posterior air sacs. In the first expiration, air moves into the lungs. On the second inspiration, air is drawn from the lungs into the anterior air sacs. In the second expiration, air is exhaled to the outside. To meet the rigorous demands of air consumption in flight, birds have a highly efficient one-way air flow through the lungs and air sacs. Mammals have a cul-de-sac system where old air in the lungs is mixed with the new. Muscle movement of the ribs and sternum creates the vacuum needed for air circulation in and out of the lungs and air sacs. Pneumatic bones are also connected to the air sacs. Scientists have shown that birds can actually "breathe" through a few of the larger pneumatic bones if they're broken.
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