Birds are endothermic; that is, they generate heat to warm the body internally. Rapid breathing and digestion of large quantities of food support the high metabolic rate necessary to generate this heat. Birds, unlike reptiles, cannot go for long periods without eating. To help conserve body heat, birds may fluff out their feathers. Aquatic birds have a thin layer of fat that provides additional insulation.
A bird's digestive and excretory systems are adapted for the rapid processing of food and metabolic wastes. The high energy requirements of flight make this efficient system necessary. For example, the carnivorous shrike can digest a mouse in three hours figure 42-5
Word Roots and Origins proventriculus from the Greek pro, meaning "before," and the Latin venter, meaning "belly"
The high amount of energy required to fly and regulate body heat is obtained by a quick and efficient digestive system, as illustrated in Figure 42-5. Because birds do not have teeth, they are not able to chew their food. Instead, food passes from the mouth cavity straight to the esophagus. An enlargement of the esophagus called the crop stores and moistens food. Food then passes to the two-part stomach. In the first chamber, the proventriculus (PROH-ven-TRIK-yoo-luhs), acid and digestive enzymes begin breaking down the food. Food then passes to the gizzard, the muscular portion of the stomach, which kneads and crushes the food. The gizzard often contains small stones that the bird has swallowed. These aid in the grinding process. Thus, the gizzard performs a function similar to that of teeth and jaws. Seed-eating birds usually have a larger crop and gizzard—relative to body size—than meat-eating birds do.
From the stomach, food passes into the small intestine. There, bile from the liver and enzymes from the pancreas and intestine further break down the food. The nutrients are then absorbed into the bird's bloodstream. Passage of food through the digestive system of a bird is usually very rapid. For instance, a thrush can eat blackberries, digest them, and excrete the seeds 45 minutes later.
The avian excretory system is efficient and lightweight. Unlike other vertebrates, most birds do not store liquid waste in a urinary bladder. The two kidneys filter a nitrogenous waste called uric acid from the blood. Concentrated uric acid travels through ducts called ureters to the cloaca, where it mixes with feces and is then excreted. This system is adaptive for flight because birds do not need to carry much water in their bodies.
The unique architecture of the bird's respiratory system provides a constant flow of oxygenated air to the lungs. This highly efficient system allows birds to maintain the high metabolic rate necessary for flight. It also enables birds to function at high altitudes, where other animals would suffer from the low availability of oxygen.
The high metabolic rate of birds requires large amounts of oxygen. Yet some birds migrate thousands of miles at altitudes as high as 7,000 m (23,000 ft), where air pressure is very low. So, birds have an elaborate and highly efficient respiratory system. Air enters the bird's body through paired nostrils located near the base of the beak. The air passes down the trachea and enters the two primary bronchi. From the bronchi, some of the air moves to the lungs. However, about 75 percent of the air bypasses the lungs and flows directly to posterior air sacs, shown in Figure 42-6. In most birds, nine sacs extend from the lungs and occupy a large portion of the bird's chest and abdominal cavity. These sacs also extend into some of the long bones. Thus, the air sacs not only function in respiration but also greatly reduce the bird's density.
Gas exchange does not occur in the air sacs. Their function is to store and redirect air. When the bird exhales, the oxygen-poor air from its lungs is forced into the anterior air sacs, and the oxygen-rich air in the posterior air sacs is forced into the lungs. This way, the bird has oxygenated air in its lungs during both inhalation and exhalation.
The avian circulatory system has characteristics that are similar to those of either reptiles or mammals or both. Like crocodiles and mammals, birds have a four-chambered heart with two separate ventricles. Deoxygenated blood is always kept separate from oxygenated blood. In comparison with most other vertebrates, most birds have a rapid heartbeat. A hummingbird's heart beats about 600 times a minute. An active chickadee's heart beats 1,000 times a minute. In contrast, the heart of the larger, less active ostrich averages 70 beats per minute, or about the same rate as a human heart. Avian red blood cells have nuclei.
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