Birds Began as Feathered Reptiles
Birds evolved during the great reptilian radiation of the Mesozoic era. Amniotic eggs and scales on the legs are just two of the reptilian features we see in birds. But modern birds look quite different from modern reptiles because of their feathers and other distinctive flight equipment.
Characteristics of Birds
Almost every part of a typical bird's anatomy is modified in some way that enhances flight. The bones have an internal structure that is honeycombed, making them strong but light. The skeleton of a frigate bird, for instance, has a wingspan of more than 2 meters but weighs only about 113 grams. Another adaptation reducing the weight of birds is the absence of some organs. Females, for instance, have only one ovary. Also, modern birds are toothless, an adaptation that trims the weight of the head. Food is not chewed in the mouth but ground in the gizzard, a digestive organ near the stomach. (Crocodiles also have gizzards, as did some dinosaurs.) The bird's beak, made of keratin, has proven to be very adaptable during avian evolution, taking on a great variety of shapes suitable for different diets.
Flying requires a great expenditure of energy from an active metabolism. Birds are endothermic; they use their own metabolic heat to maintain a warm, constant body temperature. Feathers and, in some species, layers of fat provide insulation that enables birds to retain their metabolically generated heat. An efficient respiratory system and a circulatory system with a four-chambered heart keep tissues well supplied with oxygen and nutrients, supporting a high rate of metabolism. The lungs have tiny tubes leading to and from elastic air sacs that help dissipate heat and reduce the density of the body.
For safe flight, senses, especially vision, must be acute. Birds have excellent eyes, perhaps the best of all the vertebrates. The visual areas of the brains are well developed, as are the motor areas; flight also requires excellent coordination.
With brains proportionately larger than those of reptiles and amphibians, birds generally display very complex behavior. Avian behavior is particularly intricate during breeding season, when birds engage in elaborate rituals of courtship. Because eggs are shelled when laid, fertilization must be internal. Copulation involves contact between the mates' vents, the openings to their cloacae. After eggs are laid, the avian embryo must be kept warm through brooding by the mother, father, or both, depending on the species.
A bird's most obvious adaptation for flight is its wings. Birds' wings are airfoils that illustrate the same principles of aerodynamics as the wings of an airplane. Providing power for flight, birds flap their wings by contractions of large pectoral (breast) muscles anchored to a keel on the sternum (breastbone). Some birds, such as eagles and hawks, have wings adapted for soaring on air currents and flap their wings only occasionally; other birds, including hummingbirds, must flap continuously to stay aloft. In either case, it is the shape and arrangement of the feathers that form the wings into an airfoil. The fastest birds are the appropriately named swifts, which can fly 170 km/hr.
In being both extremely light and strong, feathers are among the most remarkable of vertebrate adaptations. Feathers are made of keratin, the same protein that forms our hair and fingernails and the scales of reptiles. Feathers may have functioned first as insulation during the evolution of endothermy, only later being co-opted as flight equipment.
The evolution of flight requires radical alteration in body form, but flight provides many benefits. It enhances hunting and scavenging: many birds exploit flying insects, an abundant, highly nutritious food resource. Flight also provides ready escape from earthbound predators and enables some birds to migrate great distances to utilize different food resources and seasonal breeding areas. The bird that travels farthest in its annual migration is the arctic tern, which flies round-trip between the North Pole and South Pole each year.

单选题 According to paragraph 2, how did birds adapt to achieve efficient flight?
A. They developed a skeleton with fewer bones.
B. Their organs became smaller over time.
C. Most of their weight was distributed in their heads.
D. Teeth were replaced by a beak made of keratin.

单选题 The word "modified" in the passage is closest in meaning to ______.
A. made different B. made better
C. made smaller D. made modern

单选题 The word "their" in the passage refers to ______.
A. feathers B. species C. layers D. birds

单选题 The word "elaborate" in the passage is opposite in meaning to ______.
A. simple B. quiet C. sad D. short

单选题 According to paragraph 6, which of the following is true about the wings of birds? ______
A. All birds flap their wings constantly by using breast muscles.
B. Eagles and hawks have wings that propel thereat 170 km/hr.
C. The airfoils of birds function like the wings on airplanes.
D. Wings are attached to airfoils in the bird's skeletal structure.

单选题 In paragraph 7, the author explains the term "keratin" by ______.
A. identifying it in hair and fingernails
B. comparing it to bird feathers
C. providing a definition in the text
D. describing the way that it looks

单选题 The word "radical" in the passage is closest in meaning to ______.
A. very clear B. very wide C. very fast D. very new

单选题 According to the passage, which characteristic do birds share with reptiles?
A. They defend themselves with claws.
B. They reproduce by laying eggs.
C. They have a similar skeletal structure.
D. They utilize a two-chambered head.

单选题 Which of the sentences below best expresses the information in the highlighted statement in the passage? The other choices change the meaning or leave out important information.
A. Very early birds appeared after the dinosaurs.
B. Birds and dinosaurs may share a mutual ancestor.
C. Birds and dinosaurs were probably very common.
D. The origin of birds and dinosaurs is very early.

单选题 According to paragraph 9, what can be inferred about research on the evolution of birds? ______
A. The research is not very recent.
B. The subject is popular.
C. Many researchers are working on it.
D. There are some definitive conclusions.

单选题 All of the following are mentioned as adaptations to the bird's anatomy to accommodate flight EXCEPT ______.
A. a covering of feathers B. relatively large brains
C. very sharp eyes D. small legs and feet

单选题 Look at the four squares, , , and

填空题 Directions: An introduction for a short summary of the passage appears below. Complete the summary by selecting the THREE answer choices that mention the most important points in the passage. Some sentences do not belong in the summary because they express ideas that are not included in the passage or are minor points from the passage. This question is worth 2 points.
Birds evolved during the reptilian radiation of the Mesozoic era.
·______
·______
·______

A. Birds and reptiles are most probably related.
B. Feathers are among the most unusual evolutionary changes.
C. Many structural adaptations were required for birds to fly.
D. Theropods are relatively small, meat-eating dinosaurs.
E. There are a number of advantages for creatures that fly.

F. Migration patterns are typical of many species of birds.