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 1800s. Another thirty percent
occurred in the 1900s. Over ninety percent of these extinctions 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 its part too.
The respiratory system in birds serves to transfer oxygen to the birds
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 birds esophagus their is a bag-like
swelling called the crop. Birds 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 birds vent in the rear of the body.
The circulatory system distributes blood through the birds body. The
heart of a large bird, like an ostrich, beats approximately the same rate of a
humans 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 birds heart.
A birds nervous system consists basically of nerves and a brain. Nerves
carry messages from a birds senses to the brain, and from the brain to the
muscles. This provides a reaction to something. On a birds 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 females 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.