MODERN BIRDS ARE A VERY SUCCESSFUL GROUP

Date: 2015-10-07; view: 433.

A. Read the text below using a dictionary.

II. Reading Activities.

С. Match the following words with their definitions.

B. Look up in the dictionary the meaning of the following terms and memorize them.

A. Practise the pronunciation of the following words and memorize them.

I. Pre-reading Activities.

• muscle /ˈmʌs(ə)l/ • feather /ˈfɛðə/ • gizzard /'gɪzəd /

• digestive /daɪ'ʤestɪv/ • feature /ˈfiːtʃə/ • tissue /'tɪʃu:/

• plumage /ˈpluːmɪdʒ/ • species /'spi:ʃi:z / • rigidity /rɪ'ʤɪdətɪ /

• proventriculus • nitrogenus

• endotherms • semisolid

• avian • bladder

• esophagus • gastric juice

1. generate A. produce or cause something

2. reflection B. an image that you can see in a mirror, glass or water

3. refraction C. get rid of waste material from your body

4. excrete D. when light changes direction as it passes through glass, water

5. provide E. give something to somebody

6. possess F. have a particular quality or ability

7. remain G. become larger in size, number

8. expand H. stay in the same place

D. Discuss the questions.

- When you compare a bird and a reptile there are many obvious differences. Yet they may be descended from a common ancestor. What ancestor do they have in common?

- What is a reptile? What is a bird?

- Say in which ways birds and reptiles are similar and in which ways they differ.

Encyclopedia of animals

Even modern birds possess some characteristics in common, with the reptiles. For example, they lay eggs and have reptilian-type scales on their legs. About 9000 species of birds have been described, and they have been classified into 30 orders. Birds inhabit a wide variety of habitats and can be found on all of the continents, most islands, and even the open sea. The large living birds are the ostriches of Africa, which may be up to 2 m tall and weigh 136 kg, and the great condors of the Americas, with wingspans of up to 3 m. The smallest known bird is Helena's hummingbird of Cuba, with a length of less than 6 cm and a weight of less than 4 g.

The anterior limbs of birds are wings, usually modified for flight. The posterior limbs are modified for walking, swim­ming, or perching. Not all birds fly. Some, such as penguins have small, flipper-like wings used in swimming. Others, such as the ostrich and cassowary, have only vestigial wings, but well developed legs.

In addition to feathers and wings, birds have many other adaptations for flight. Their bodies are compact and streams-lined., and the fusion of many bones gives them the rigidity needed for flying. Their bones are strong but very light; many are hollow, containing large air spaces. The avian jaw is light and, instead of teeth, there is a light, horny beak. The breastbone is broad and flat for the attachment of the large flight muscles.

Birds have efficient lungs with thin-walled extensions, called air sacs, that occupy spaces between the internal organs and within certain bones. Like mammals, birds have a four-chambered heart and a double circuit of blood flow. Blood delivers oxygen to the tissues and then is recharged with oxygen in the lungs before being pumped out into the systemic circulation again. The very effective respiratory and circulatory systems provide the cells with enough oxygen to permit a high metabolic rate, which is necessary for the tremendous muscular activity that flying requires. Some of the heat generated by metabolic activities is used to maintain a constant body temperature. Birds and mammals are endotherms.Their ability to maintain a constant body temperature permits birds to re­main active in cold climates.

Birds excrete nitrogenous wastes mainly as semisolid uric acid. Because they lack a urinary bladder, these solid wastes are delivered into the cloaca. They leave the body with the fe­ces, which are dropped frequently. This adaptive mechanism helps to maintain a light body weight.

Birds have become adapted to a variety of environments, and various species have very different types of beaks, feet, wings, tails, and behavioral patterns. Bills are specifically adapted for the type of food the bird eats. Although all birds must eat frequently (because they have a high metabolic rate and do not store much fat), the choice of food varies widely among species. Most birds eat energy-rich foods such as seeds, fruits, worms, mollusks, or arthropods. Warblers and some other species eat mainly insects. Owls and hawks eat rodents, rabbits, and other small mammals. Vultures feed on dead an­imals. Pelicans, gulls, terns, and kingfishers catch fishes. Some hawks catch snakes and lizards.

An interesting feature of the bird digestive system is the crop, an expanded, saclike portion of the digestive tract below the esophagus, in which food is temporarily stored. The stom­ach is divided into a proventriculus, which secretes gastric juices, and a thick, muscular gizzard, which grinds food. The bird swallows small bits of gravel that act as "teeth" in the giz­zard, mechanically breaking down food.

Birds have a well developed nervous system with a brain that is proportionately larger than that of reptiles. Birds rely heavily on vision, and their eyes are relatively larger than those of other vertebrates. Hearing is also well developed.

In striking contrast to the relatively silent reptiles, birds are very vocal. Most have short, simple calls that signal danger or influence feeding, flocking, or interaction between parent and young. Songs are usually more complex than calls and are performed mainly by males. Songs are related to repro­duction, attracting and keeping a mate, and claiming and de­fending territory.

One of the most fascinating aspects of bird behavior is the annual migration that many species make. Some birds, such as the golden plover and Arctic tern, fly from Alaska to Patagonia, South America, and back each year, covering perhaps 40,250 km (25,000 mi) en route.

Beautiful and striking colors are found among birds. Color is due partly to pigments deposited during the development of the feathers and partly to reflection and refraction of light of certain wavelengths. Many birds, especially females, are pro­tectively colored by their plumage. Brighter colors are often assumed by the male during the breeding sea­son to help in attracting a mate.

III. Speaking Activities.