Class Aves: Birds

Birds are warm–blooded animals with complete double circulation and a skin covered with feathers. The forelimbs are wings. Flight is not a feature that makes birds unique, however. Birds share this ability with bats and insects. Birds, like the reptiles, fertilize their eggs internally and lay eggs with shells that do not dry out on land. Their embryo is enclosed in an eggshell with protective membrane. About 8.590 species are known. In their skeleton structure birds are essentially reptiles highly specialized for flight. They have feathers, however, one of the distinctions of this class, and unlike the reptiles, they maintain a high and constant body temperature, which may in part permit the high-energy output required for flight. Their bodies are lightened by air sacs and also by hollow bones.

The most massive bone in the bird skeleton is the breastbone, or sternum, which provides the keel for the attachment of the huge muscles that operate the wings. Flying birds have jettisoned all extra weight, the female reproductive system has been trimmed down to a single ovary, and this becomes large enough to be functional only in the mating season. The pectoralis muscle lowers the wing and the supracoracoideus raises it. A group of vertebrae, called sacral vertebrae, are located at the lower end of the spine. The femur is the upper leg bone, the lower leg bone is the tibia.

Most of the common nonflying birds such as the penguin and the ostrich are believed to have evolved secondary from flying types.

Characteristics of birds:

· The body is divided into neck, head, trunk and tail;

· Bones are lightweight and filled with air;

· Air sacs are found throughout the body;

· Birds lack teeth; jaws are covered by horny beak;

· Front limbs are modified into wings; hind limbs are adapted for perching, hopping, swimming or other similar activity;

· Body temperature remains constant;

· The heart has four chambers;

· Reproduction involves the production of an amniotic egg most species incubate in a nest.

One big difference between birds and all the other animals you have studies so far is that birds are warm-blooded. The body temperature of cold-blooded animals is about the same as their surroundings. So, cold-blooded animals can not live where the air temperature stays below the freezing point of water for long periods of time. The body temperature of the warm-blooded animals does not depend on their surroundings. Their body temperature always stays about the same. A temperature is about 41 degrees of C is normal for birds. Birds keep this high temperature because they have a high rate of metabolism. That is they turn food into energy rapidly. But birds also need a lot of energy because they are active. They also need a lot of energy to keep their body temperature up – especially in cold weather. Because in their high rate of metabolism birds eat a very large amount of food for their size. In fact, some birds eat their own weight in food each day.

The ways birds are different and similar

Birds differ from other vertebrates in that they have feathers. There are important differences among birds. Birds are classified on the basis of a number of characteristics, including body structure and behavior, into 4 common types: flightless birds, water birds, perching birds, birds of prey. Each group of birds is adapted to a specific way of life. Legs, feet, and beaks differ according to the ecological role of the group. Perching birds that live in trees have curved toes that are able to cling to branches. Predators capture their prey with sharp claws called talons. The foot of a duck is suited for paddling and swimming. The foot of the hawk is suited for grasping prey. The beaks of birds also vary widely in their use and structure. The thick beaks of some finches are suited for cracking seeds. An eagle’s beak is adapted to cutting and tearing flesh. A woodpecker’s beak is used for drilling through wood.

Many birds are especially helpful for humans because they eat the insects and rodents that damage food crops. People often enjoy watching or photographing different species of birds. Nest-building chores are of particular interest to many bird-watchers. Birds can also be harmful to human. Some birds carry diseases, such as influenza. Many species of birds eat our valuable crops. Some birds, such as pigeons, foul buildings and statues. Pigeons have also been known to peak away the material between bricks. This material contains lime that the pigeons use in their eggs.

Flight

Most birds fly. The structure of birds is well suited for flight. Their skeleton, for example, is light in weight. Yet it is very strong. Their bones are hollow and have thin walls. Flying requires a very good circulatory system to get large amounts of food and oxygen to all the cells of the body.

The wings are very important in flight. The part of the bird’s wing that is closest to its body is shaped like an airplane wing. This shape - called airfoil – gives each wing an upward push, which the bird needs to stay in the air. The upward push – called lift – is made possible because air moves faster over the wing than under the wing. The wings not only provide lift but also provide thrust, the force needed to move the bird forward. A bird has large, powerful breast muscles for moving its wings. In most birds, these powerful muscles are able to work steadily for long periods of time.

Some birds, such as swallows and gulls, have wings that are best for continuous flight. Other birds, such as quail and pheasant, have wings that are best for short, rapid flight. Still other birds, such as ostriches, are flightless. They cannot use their wings for flight at all.

A bird’s feathers are its plumage. Different species of birds can be recognized by the colors and designs of their plumage. Males of a species are often more brightly colored than females, as shown by the pheasant pair. The duller color of the female can help her blend in with her surroundings when she hatches eggs and cares for her young.

Feathers are outgrowths of the bird’s skin that facilitate flight and also provide insulation. Feathers evolved from the scales of birds’ reptile ancestors. Feathers grow from small sacs in the skin called follicles. They first appear as small, dark pin feathers that have a scaly covering. Mature feathers are strong and light because of their unique structure. Feathers have a central cylinder or rachis running their length. The part of the rachis that extends out of the skin is solid, while that beneath the skin is hollow. This hollow section is known as the quill. Birds have 4 types of feathers: contour, down, filoplumes, and bristles. Contour feathers are smooth, sleek feathers that cover the head, body and wings. These feathers protect and streamline the bird. Down feathers are soft, fluffy feathers found under contour feathers. Down feathers insulate birds. Filoplumes are short, thin feathers that look like hair. Bristles, also short, hair-like feathers, are located near the bird’s nostrils. Bristles filter dust.

Digestive system. Birds must eat large quantities of food to generate the energy needed for both flight and temperature regulation. Different species of birds have different diets. The digestive system of a bird processes a large amount of food at a time. Food is taken in through the mouth, travels down the esophagus, and enters a large crop, which is used primarily for food storage. Upon leaving the crop, food moves into a two-chambered stomach. The first chamber is the thick-walled proventriculus where food is mixed with digestive enzymes. The second chamber of the stomach is the gizzard. The gizzard functions as a substitute for teeth; it grinds up the food. A bird’s gizzard has a hard, horny lining. The duodenum, the coiled intestine and the other digestive organs of the bird are similar to those in other vertebrates

Respiratory system

A bird’s high body temperature and level of activity require a large amount of oxygen. Air is drawn into the body through the mouth or the two nostrils in the beak. The air then passes through the trachea into a pair of tubes called bronchi. Each bronchus leads to one of the lungs. Much of the inhaled air travels into air sacs – cavities that make the bird lighter when filled with air. Air sacs extend into the body cavity and even into the toes. Because of the air sacs, a bird’s respiratory system may take as much as 20 percent of the body volume. Some of the air in the body is used for making sounds. Birds do not have vocal cords. They produce their calls instead by means of their syrinx, or “song box”. The syrinx is located at the bottom of trachea. It contains a pair of vibrating membranes. Muscles attached to the membrane change the pitch of the sound.

Circulatory system

Birds have four-chambered hearts. The right and left sides of the heart are completely separate. Each side of the heart is divided into two chambers, an upper atrium and a lower ventricle. The right side of the heart receives oxygen-poor blood from all parts of the body and pumps it to the lungs. After the blood is oxygenated in the lungs, it travels to the left side of the heart. From there it is pumped out to the body through the aortic arch. A bird’s heart beats very rapidly, although the rate varies greatly from species to species. A mourning dove’s heart rate ranges between 135 and 570 beats a minute. Thus, the blood circulates rapidly. Rapid circulation is important in very active animals like birds.

Excretory system

The excretory system of a bird is modified to converse water. Almost all the water is removed from urinary products as they pass from kidneys to the cloaca. The primary form of nitrogenous wastes is a white solid compound called uric acid. Wastes from the intestine and from the kidneys are excreted together.

Nervous system

The nervous system of a bird is highly developed. The brain has three well-developed areas: the cerebellum (controls flying and walking by coordinating movement), the midbrain tectum (the center of vision), the optic lobes (the place where visual impulses are interpreted is located in this area). Birds have keen senses of hearing and balance but poor senses of smell and taste. They depend more on their vision than on any other sense. Birds have extremely large eyes that detect slight movements. Although the eyes are unable to rotate, a bird still has a wide range of vision because its neck is flexible due to a large number of vertebrae.

Reproductive system. Male birds produce sperm in testes. The mature sperm pass from the testes into a long tube, called the vas deferens. The lower part of the vas deferens is enlarged to form a seminal vesicle where the sperm are stored until mating occurs. The female of most bird species has only a single ovary, located on the left side. Eggs produced in the ovary are released into the body cavity. After mating occurs and sperm fertilize the eggs, the eggs enter the oviduct. The oviduct is a long tube that leads to the cloaca. As eggs travel through the oviduct they are covered by albumin, a nutritious protein that makes up the egg white. Later, the eggs are covered by shells made primarily of calcium carbonate, also called lime. Eggs with shells leave the body through the cloaca. Eggs that are not fertilized also receive albumin and a shell.

Migration. Many bird species mate, build nests, and raise their young in one part of the world and then move to another to avoid winter. A common sign of winter’s approach is the sight of birds overhead, flying south. Similarly, the return of birds to the north signals the coming of spring. This regular, seasonal movement is known as migration. These trips are often long and difficult. Year after year birds migrate over the same paths, called migration routes. Birds find their way partly by instinct and partly by learned behavior.

5.9.6. Class Mammalia.Mammals

Characteristics of Mammals

The 4,500 species of mammals live throughout the world. Mammals can live in different environments because their flexible body plan has allowed the various species to undergo many special adaptations.

The class Mammalia consists of vertebrates whose young are nourished by their mother’s milk. They have four-chambered hearts. Also, many mammals are swift and agile because their legs are more directly under their bodies. Mammals include all the most familiar domesticated animals, such as dogs, cats, horses, and cows. Mammals also include human beings, as well as many animals that still live in the wild.

Fossil evidence indicates that mammals evolved more than 180 million years ago. Although many of the first species of mammals died out, some developed into forms that are known today. Most of these mammals had come into existence in some recognizable form.

Mammals have several characteristics not found in other vertebrates:

· The nursing of young: Mammals have modified sweat glands called mammary glands that secrete the milk used to feed the young after they are born.

· Body hair: Only mammals have protective covering of body hair. The hair acts as insulation and also protects the body from injury.

· Live birth: Most mammals are viviparous – that is, the young are born alive after developing inside the mother.

· Extended parental care: All mammalian young remain with their parents while learning to take care of themselves.

· A large well-developed brain: More than any other single organ, the brain has enabled mammals to survive change and to develop complex patterns of behavior.

· An outer ear: Mammals are the only animals that have an outer ear, which collects sound and transmits it to the middle and inner ear.

· Separate chest and abdominal cavities: In mammals, a dome-shaped muscle called the diaphragm divides the coelom into two parts.

Movement of Mammals

A mammal’s legs are directly underneath its body. Thus, little energy is required to hold the body on the ground. The elbow of the forelimb is turned backward, and the knee of the hindlimb bends forward. Also, the limbs of mammals are longer and more slender than those of reptiles. These changes have increased the speed at which some mammals can travel on land. The cheetah, for example, can reach incredible burst of speed. It can run up to 113 km per hour for short distances.

Limbs of mammals differ. Mammals have become adapted to life in many environments. Bats are mammals that can fly. Thin membrane stretch between their long fingers to form wings. Whales, dolphins and porpoises are water-dwelling mammals. Their limbs are fishlike flippers that are used for swimming. Monkeys, apes and humans are primates with well-developed fingers and toes. The limbs of primates are good for climbing, grasping and using tools.

Date: 2014-12-22; view: 1007