Birds are some of the most amazing animals on earth. Most have the ability to fly. Some use ground travel. Some use claws, others use only their beaks. Birds come in many varieties of colors and sizes. Birds are warm-blooded, egg-laying creatures from the aves class. Along with the obvious feathers and wings, birds have other adaptations for flying such as a wide keel on the sternum, with large wing muscles attached, air spaces and sacs throughout the body and bones, to decrease their weight, and they have various bone fusions and reductions to strengthen and streamline their body. There are more than 8700 species of birds. Their habitats range from icy shores of Antarctica to the hottest parts of the tropics and from mountains, deserts, plains, and facts to open oceans and deeply urbanized areas. The sizes of birds range from the tiny bee hummingbird, which has a total length of two and a half inches, to the albatross, which has a wing span of eleven and a half feet. The largest bird is a bird that cannot fly, the ostrich. Ostriches can stand almost eight feet high and can weigh near 350 pounds. Other extinct birds have been measured to stand over ten feet high. The evolution of birds is still being argued. Most people believe that birds evolved from reptiles. Because of birds mainly delicate bones, few fossils have been left behind for scientists to study. The earliest bird fossils come from archaeopteryx. The fossils that have been discovered from archaeopteryx include six partial skeletons and one single feather. Archaeopteryx , unlike modern birds, had teeth, a reptile like tail, and three claws on each wings. Scientists think it could fly, but only weakly. Approximately 85 species and 50 sub species have become extinct in the last 300 years. Over half of them occurred in the 1800's. Another thirty percent occurred in the 1900's. Over ninety percent of these extinction's were island forms, which are particularly vulnerable to human interference. Destruction of habitat is the biggest cause of extinction. Other causes are the introduction of predacious animals, and disease plays it's part too. The respiratory system in birds serves to transfer oxygen to the bird's bloodstream. Unlike mammals, birds do not have sweat glands. So they cannot cool themselves through perspiring. Air sacs throughout the body are connected to the lungs. As the bird breathes, the air sacs help cool the birds organs. The average body temperature of birds is about 106° F. Birds do not have any teeth. This means that birds must cut food up with their beaks or swallow it whole. On a bird's esophagus their is a bag-like swelling called the crop. Bird's can store food there until there is room in the stomach for it. They can also store food their for their young. In most birds, the stomach is two parts. the first part is where digestive juices are added. The second part, called the gizzard, has thick , muscular walls for grinding up food. This replaces chewing. A lot of birds help the grinding process by swallowing coarse materials like gravel. The nutritious matter is absorbed in the small intestine. Then waste matter moves on to the large intestine. All waste from birds release from the bird's vent in the rear of the body. The circulatory system distributes blood through the bird's body. The heart of a large bird, like an ostrich, beats approximately the same rate of a human's heart, 70 times a minute. Other small birds, like a hummingbird, have a heart beat of more than 1000 times a minute! Arteries in birds carry blood from the heart to organs in the body. Veins return blood to the bird's heart. A bird's nervous system consists basically of nerves and a brain. Nerves carry messages from a bird's senses to the brain, and from the brain to the muscles. This provides a reaction to something. On a bird's brain, the cerebellum is relatively larger than a cerebellum on a mammal. The cerebellum is what birds use to control balance and the muscles they use to fly. Male birds have testes and the female birds have ovaries, just like in other vertebrates. Most birds mate by pressing their vents together. Sperm cells quickly pass into the female's vent and unite with one or more egg cells. The union produces a fertilized egg, or a zygote. When the egg is laid, the zygote develops into an embryo as the egg is incubated.
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