Section 31 2 1 FOCUS Objectives 31.2.1 Describe the characteristics that all birds have in common. 31.2.2 Summarize the evolution of birds. 31.2.3 Explain how birds are adapted for flight. 31.2.4 Describe the diversity of birds. 31.2.5 Identify ways in which birds interact with the environment and with humans. Vocabulary Preview Have students compare endotherm to ectotherm. Ask: If endo- means within, how does an endotherm control its body temperature? (Endotherms generate their own body heat; they control body temperature from within.) Reading Strategy Before students read this section, have them preview Figure 31 14. Instruct them to list characteristics common to all birds. As students read, encourage them to add or subtract characteristics to complete their list. 2 INSTRUCT 31 2 Birds Key Concepts What characteristics do birds have in common? How are birds adapted for flight? Vocabulary feather endotherm crop gizzard air sac Reading Strategy: Monitoring Your Understanding As you read, make sure that you understand what you read. If you have difficulty, think of a strategy that might make the text clearer. For example, you might read the paragraph again, slowly; see whether an illustration helps you understand the printed text; or ask another student or your teacher for help. Figure 31 11 Birds have different types of feathers that vary in structure and function. An outer covering of feathers is the main characteristic that sets birds apart from other animals. Whether they are greeting the dawn with song or coloring the air with brilliant feathers, birds are among the most obvious and welcome of all animals. From common robins to the spectacular and rare quetzal of Central America, the nearly 10,000 modern bird species seem to live everywhere. What Is a Bird? In a group this diverse, it is difficult to find many characteristics that are shared by all members. But we can identify the features that most birds have in common. Birds are reptilelike animals that maintain a constant internal body temperature. They have an outer covering of feathers; two legs that are covered with scales and are used for walking or perching; and front limbs modified into wings. Most of these features are adaptations for flight. The single most important characteristic that separates birds from living reptiles, and from all other living animals, is feathers. Feathers are made mostly of protein and develop from pits in the birds skin. Feathers help birds fly and also keep them warm. Figure 31 11 shows the two main types of feathers: contour feathers and down. Herons and some other birds that live on or in water also have powder down, which releases a fine powder that repels water. Contour feather Contour feathers provide the lifting force and balance needed for flight. Barb What Is a Bird? Use Visuals Figure 31 11 Have students compare and contrast the structure and function of contour and down feathers. Ask: Which feathers insulate a bird s body? (Down) Which feathers help the bird to fly? (Contour) Discuss how the structure of down feathers makes them good for insulation. Ask: Would you expect a bird with only down feathers to fly? (No; down feathers do not give the stability needed for flight.) Print: SECTION RESOURCES Laboratory Manual B, Chapter 31 Lab Teaching Resources, Section Review 31 2, Enrichment, Chapter 31 Exploration Reading and Study Workbook A, Section 31 2 Adapted Reading and Study Workbook B, Section 31 2 Down feather Down feathers trap air close to the body and keep the bird warm. Issues and Decision Making, Issues and Decisions 13 Lesson Plans, Section 31 2 Technology: Barbule The hooks on each barbule fit together, holding them flat. itext, Section 31 2 Transparencies Plus, Section 31 2 806 Chapter 31
Modern reptiles Ornithischia (bird-hipped dinosaurs) Reptilian ancestor Evolution of Birds Paleontologists agree that birds evolved from extinct reptiles. Evidence for this hypothesis is provided by many embryological, anatomical, and physiological characteristics shared by modern birds and living reptiles. For example, the embryos of birds and reptiles develop within amniotic eggs. Birds, like most reptiles, excrete nitrogenous wastes in the form of uric acid. The bones that support the front and hind limbs, and several other parts of the skeleton, are similar in both groups. Most paleontologists think that birds evolved directly from dinosaurs. Part of the evidence consists of Archaeopteryx (ahrkee-ahp-tur-iks), the first birdlike fossil discovered. This fossil dates from the late Jurassic Period, about 150 million years ago. Archaeopteryx looked so much like a small, running dinosaur that it would be classified as a dinosaur except for one important feature: It had well-developed feathers covering most of its body. Those feathers led to the classification of Archaeopteryx as an early bird. Unlike modern birds, however, this creature had teeth in its beak, a bony tail, and toes and claws on its wings. Thus, Archaeopteryx can be seen as a transitional species with characteristics of both dinosaurs and birds. However, other fossil evidence leads some researchers to hypothesize that birds and dinosaurs both evolved from an earlier common ancestor. The origin of birds is still not completely resolved, as shown in Figure 31 12. New fossils of ancient birds are being found all the time. So watch for new discoveries and discussions on the subject! What is Archaeopteryx? Ancestor of dinosaurs Dinosaurs Saurischia (lizard-hipped dinosaurs) Archaeopteryx Modern birds Figure 31 12 The diagram at the top shows the evolutionary tree of modern birds. None of the animals shown are direct ancestors of modern birds. But fossils such as Archaeopteryx (above) do show a mixture of characteristics of birds and dinosaurs. Interpreting Graphics Based on the diagram, what are the two alternative explanations for the evolution of modern birds?? Evolution of Birds Use Visuals Figure 31 12 Have students examine Figure 31 12. Ask: In what ways was Archaeopteryx similar to modern birds? (It had feathers.) How was it similar to dinosaurs? (It had teeth in its beak, a bony tail, and toes and claws on its wings.) Why do you think many scientists infer that birds evolved from dinosaurs? (Discoveries of many birdlike dinosaur fossils, showing characteristics of both dinosaurs and birds) Inferring Remind students that many researchers believe birds evolved directly from dinosaurs, based on the evidence found in fossilized species. Challenge students to infer what kind of fossilized evidence researchers would expect to find if birds and dinosaurs had instead evolved from a common ancestor. (If birds and dinosaurs each evolved from a common ancestor, you would not expect to find any fossilized species that are intermediate between birds and dinosaurs.) Students can write a short report that describes their inference and the evidence or background information on which they based their inference. Inclusion/Special Needs Pair visually impaired students with other students and have them work together to make observations about the characteristics of birds. Make available different types of feathers, various avian bones and skulls, or any threedimensional bird model. The sighted student can identify structures as the visually impaired student tactilely examines their textures and shapes. Less Proficient Readers Students can review the process of digestion in birds by making a flowchart. Students should include every part of the digestive system in which something happens to food. They should start with the mouth and end with the cloaca. Answers to... Birdlike species with characteristics of both dinosaurs and birds Figure 31 12 Either evolved from dinosaurs or from a reptilian ancestor common to dinosaurs and birds Reptiles and Birds 807
31 2 (continued) Form, Function, and Flight Predicting Give student groups 10 to 15 pictures of various birds. Instruct groups to predict which birds fly and which do not. Next, have students predict which of the flying birds are the best fliers. After making their predictions, groups should list the characteristics they used for making their predictions. Then, tell students the answers. Encourage them to review their predictions and the criteria they used to make them. Ask groups if they would change any of the criteria they used to make their predictions and why. Make Connections Physics Let students experiment to discover how insulation helps conserve heat. Allow student groups to brainstorm a list of materials that would be good insulators. Then, have students test these materials by observing how well they can keep a warm object warm. Students might devise a plan as simple as wrapping a jar of hot water with the material and measuring the water temperature over time to determine how much heat is lost. The better the insulator, the less heat is lost. Ask: What quality determines the best insulators? (Often the best insulators trap the most air around an object.) Use Community Resources Arrange a field trip to an aviary or a display of stuffed birds at a natural history museum. Challenge students to identify the type of food each bird eats based on the shape of its bill. Northern Cardinal Pileated Woodpecker Harris s Hawk Emerald Toucan Roseate Spoonbill Hawaiian Honeycreeper Form, Function, and Flight One reason for the evolutionary success of birds is found in the adaptations that allow them to fly. Birds have a number of adaptations that enable them to fly. These adaptations include highly efficient digestive, respiratory, and circulatory systems; aerodynamic feathers and wings; strong, lightweight bones; and strong chest muscles. Most birds have these characteristics, even though some birds cannot fly. The ways in which birds carry out their life functions, such as obtaining food and oxygen, contribute to their ability to fly. For example, flight requires an enormous amount of energy, which birds obtain from the food they eat. Birds also require energy to maintain their body temperature. Body Temperature Control Unlike reptiles, which must draw body warmth from their environment, birds can generate their own body heat. Animals that can generate their own body heat are called endotherms. Endotherms, which include birds, mammals, and some other animals, have a high rate of metabolism compared to ectotherms such as reptiles. Recall that metabolism is the sum of chemical and physical processes that go on inside the body. Metabolism produces heat. A bird s feathers insulate its body enough to conserve most of its metabolic energy, allowing the bird to warm its body more efficiently. The body temperature of most birds is about 41 C even on cold winter days. What is an endotherm? Feeding Any body heat that a bird loses must be regained by eating food. The more food a bird eats, the more heat energy its metabolism can generate. Because small birds lose heat relatively faster than large ones, small birds must eat more, relative to their body size. In fact, the phrase eats like a bird is quite misleading, because most birds are voracious eaters! As you can see in Figure 31 13, birds beaks, or bills, are adapted to the type of food they eat. Insect-eating birds have short, fine bills that can pick ants and other insects off leaves and branches, or can catch flying insects. Seed-eaters have short, thick bills. Carnivorous birds, such as eagles, shred their prey with strong hooked bills. Long, thin bills can be used for gathering nectar from flowers or probing soft mud for worms and shellfish. Large, long bills help birds to pick fruit from branches, while long, flat bills are used to grasp fish. Figure 31 13 Bird bills come in a variety of shapes and sizes. You can tell a good deal about a bird s feeding habits from its bill. Drawing Conclusions Based on the size and shape of its bill, what does a roseate spoonbill feed on? TEACHER TO TEACHER TEACHER TO TEACHER Teachers can use skeletal features to demonstrate the similarities between reptiles and birds. Display skeletons of a typical lizard, a pigeon, and a plaster model of Archaeopteryx lithographica. Lead students through a discussion of typical reptilian skeletal characteristics toothed jaws, lizardlike tail, and so forth and indicate where they are found in Archaeopteryx. Finally, show where the various characteristics persist in such modern birds as the pigeon and indicate characteristics that have been lost, such as teeth. William C. Alexander, Chairman Division of Science and Mathematics SC Governor s School for Science and Mathematics Hartsville, SC 808 Chapter 31
Address Misconceptions 1 Esophagus Crop When a bird eats, food moves down the esophagus and is stored in the crop. Liver Brain Lung Figure 31 14 Birds have a number of adaptations that enable them to fly, including an efficient digestive system. Trace the path of food through the digestive system. Heart Kidney Air sac Students might think birds do not eat large amounts of food. Many have heard the phrase eats like a bird in reference to someone who doesn t eat much food. Emphasize that because birds require energy to support their active, flying lifestyle and to maintain a constant, high body temperature, they have a very high metabolic rate. To fill their energy needs, birds eat almost constantly. 2 Moistened food passes to the stomach, a two-part chamber. The first chamber secretes acid and enzymes. The partially digested food moves to the second chamber, the gizzard. 3 First chamber of stomach Gizzard Small intestine The muscular walls of the gizzard squeeze the contents, while small stones grind the food. The digestive system of a bird is shown in Figure 31 14. Birds lack teeth, and therefore they cannot break down food by chewing it. However, many birds have specialized structures to help digest food. One such structure is the crop, which is located at the lower end of the esophagus. Food is stored and moistened in the crop before it moves further in the digestive tract. In some birds, such as pigeons, the crop has a second function. During nesting season, the breakdown of cells in the crop produces a substance that is rich in protein and fat. Parent birds regurgitate this substance and feed their newly hatched young with it. This substance provides the young birds with materials they need to grow. From the crop, moistened food moves into the stomach. The form that a bird s stomach takes depends on the bird s feeding habits. Birds that eat meat or fish have an expandable area in which large amounts of soft food can be stored. Birds that eat insects or seeds, however, have a muscular organ called the gizzard that helps in the mechanical breakdown of food by grinding it. The gizzard forms part of the stomach. In many species of bird, the gizzard contains small pieces of stone and gravel that the bird has swallowed. The thick, muscular walls of the gizzard grind the gravel and food together, crushing food particles and making them easier to digest. Food moves from the stomach to the small intestine, where the breakdown of food is completed and food is absorbed into the body. Digestive wastes leave the body through the cloaca. Cloaca 4 Pancreas Large intestine 5 As digestion continues, the food moves through the intestines. Undigested food is expelled through the cloaca. NSTA For: Links on birds Visit: www.scilinks.org Web Code: cbn-9312 Use Visuals Figure 31 14 Have students trace the path of food through the bird s digestive system. Ask: What happens to food in the crop? (It s stored and moistened.) What is the function of the gizzard? (It grinds and crushes food.) Remind students that not all birds have gizzards. Ask: Which would be most likely to have a gizzard: insect-eating birds or fish-eating birds? (Insect-eating birds) Why do birds need the gizzard and the crop? (They don t have teeth. These organs help prepare food for digestion.) NSTA Download a worksheet on birds for students to complete, and find additional teacher support from NSTA SciLinks. Specialized digestive systems As in other vertebrates, a bird s digestive system is slightly different depending on its diet. Birds do not have teeth and cannot chew their food into smaller pieces. Birds that eat small animals either tear off small pieces with their bills or swallow their food whole. These birds usually do not have a crop at the end of the esophagus, because the food does not have to be softened. Their stomachs usually have only one compartment. They do not have a gizzard. Birds that primarily eat seeds usually have a crop. The crop is required to soften the hard seeds, making them susceptible to digestive enzymes. These birds often have a two-part stomach that includes a gizzard. Answers to... Animal that generates its own body heat Figure 31 13 Fish Reptiles and Birds 809
31 2 (continued) Using Models To help all students visualize the flow of air through bird lungs, have the students act as air molecules moving through the respiratory system. Use masking tape to delineate the anterior air sacs, lungs, posterior air sacs, and trachea. Students carrying red paper circles (O 2 ) enter the trachea upon inhalation 1 and move into posterior air sac. During exhalation 1, students move into lungs and exchange red circles for blue circles (CO 2 ). During inhalation 2, new red students enter posterior air sac. The blue students move to anterior air sac. During exhalation 2, the blue students move out into the air and the red students move into the lungs and exchange gases. Continue in this manner until all students have moved through the respiratory system. Drawing Conclusions Divide the class into small groups that represent a mixture of academic abilities. Groups should discuss what they know about the form and function of birds. Have one group member take notes during the discussion. Then, instruct groups to summarize how specific body structures and systems make birds adapted for flight. Suggest that groups also discuss what makes some birds able to fly while others cannot. Groups should summarize their conclusions and present them to the class. Compile the groups conclusions into a table that you can update as you complete the section. Trachea Air sacs Air sacs Lung Figure 31 15 Birds have a unique respiratory system. Air sacs direct air through the lungs in an efficient, one-way flow. Comparing and Contrasting How does this system differ from that of most land vertebrates? Figure 31 16 To keep blood moving rapidly, a bird s heart beats quickly from 150 to more than 1000 beats per minute! Applying Concepts Why is it important for a bird s heart to move blood so rapidly? Domestic pigeon Respiration Birds have a unique and highly efficient way of taking in oxygen and eliminating carbon dioxide. When a bird inhales, most air first enters large posterior air sacs in the body cavity and bones. Observe the air sacs in Figure 31 15. The inhaled air then flows through the lungs. Air travels through the lungs in a series of small tubes. These tubes are lined with specialized tissue, where gas exchange takes place. The complex system of air sacs and breathing tubes ensures that air flows into the air sacs and out through the lungs in a single direction. The one-way flow constantly exposes the lungs to oxygen-rich air. Contrast this to the system found in most land vertebrates, in which oxygen-rich air is inhaled, and oxygen-poor air is exhaled. The air travels in two directions, in and out. In an in-out system, the lungs are exposed to oxygenrich air only during inhalation. What advantage does the efficient respiratory system of birds provide? The constant, one-way flow of oxygen-rich air helps birds maintain their high metabolic rate. Birds need a high metabolism to maintain body temperature and provide the large amounts of energy required for flight. In addition, the efficient extraction of oxygen enables birds to fly at high altitudes where the air is thin. How is their respiratory system advantageous to birds? Circulation Birds have four-chambered hearts and two separate circulatory loops. Notice in Figure 31 16 that a bird s heart, unlike that of amphibians and most reptiles, has two separate ventricles, the right ventricle and the left ventricle. There is complete separation of oxygen-rich and oxygen-poor blood. One half of the heart receives oxygen-poor blood from the body and pumps this blood to the lungs. Oxygen-rich blood returns to the other side of the heart to be pumped to the rest of the body. This double-loop system ensures that oxygen collected by the lungs is distributed to the body tissue with maximum efficiency. Heart Right atrium Right ventricle From body To body From lungs To lungs Left atrium Left ventricle Complete division Bird sighted flying above the clouds Some of the highest-flying birds are bar-headed geese. These geese have been observed flying over the Himalayas, which reach altitudes of over 7600 meters. Birds are able to fly at such high altitudes because of the efficiency of their lungs. In contrast, humans have difficulty climbing Mount Everest because they inhale less oxygen due to the lower air pressure. Not only do bird lungs function efficiently at gas exchange, but the air sacs in lungs also help birds to lose heat. The air sacs are located among the bird s internal organs, and they are connected to the bones. Bird bones have hollow spaces to reduce mass but also function to hold air. As air moves across the organs and through the bones during the process of respiration, it works to cool the bird s body. 810 Chapter 31
How do birds breathe? Materials 6 round balloons, hand-powered balloon pump, measuring tape, clock with second hand Procedure 1. Work in groups of three. Make a copy of the data table at right on a blank sheet of paper. One person will inflate a balloon by mouth, while a second person inflates a balloon with a handpowered pump. The third person is the timekeeper. 2. Begin inflating both balloons at the same time. After 10 seconds, the timer will say stop. Stop inflating the balloons and pinch the necks of the balloons to keep the air inside. CAUTION: Do not try to inflate balloons by mouth if you have a condition that would make this dangerous for you. 3. Measure and record the circumference of each balloon in your data table. CAUTION: Discard all balloons that have been inflated. Name Excretion The excretory systems of many birds are similar to those of most living reptiles. Nitrogenous wastes are removed from the blood by the kidneys, converted to uric acid, and deposited in the cloaca. There, most of the water is reabsorbed, leaving uric acid crystals in a white, pasty form that you may recognize as bird droppings. Response Birds have well-developed sense organs, which are adaptations that enable them to coordinate the movements required for flight. Birds also have a brain that can quickly interpret and respond to a lot of incoming signals. A bird s brain, shown in Figure 31 17, is relatively large for its body size. The cerebrum, which controls such behaviors as flying, nest building, care of young, courtship, and mating, is quite large. The cerebellum is also well developed, as you might expect in an animal that uses precise, coordinated movements. The medulla oblongata coordinates basic body processes, such as the heartbeat. Birds have extraordinarily well developed eyes and sizable optic lobes in the brain. Birds see color very well in many cases, better than humans. Most bird species can also hear quite well. The senses of taste and smell, however, are not well developed in most birds, and the olfactory bulbs in a bird s brain are small. Data Table Balloon Circumference (cm) Mouth 4. Repeat Steps 1 3 until each member of your group has inflated two balloons. In your data table, record the difference in balloon diameter for each person and the average difference for the group. Analyze and Conclude 1. Analyzing Data Which method was faster? Which method required more effort? 2. Using Models Which method worked like reptile lungs? Which method worked like bird lungs? Explain your answers. 3. Formulating Hypotheses How is efficient respiration especially valuable to birds? Figure 31 17 Compared to reptiles, birds have an enlarged cerebellum that coordinates the movements of wings and legs. Formulating Hypotheses Why would the cerebrum also be larger in birds than in reptiles? Olfactory bulb Cerebrum Optic lobe Cerebellum Medulla oblongata Spinal cord Pump Average Difference Difference Objective Students will be able to use models to explain how birds breathe. Skills Focus Using Models, Measuring Materials 6 round balloons, handpowered balloon pump, measuring tape, clock with second hand Time 15 minutes Advance Prep Inexpensive balloon pumps are available at party supply stores. Strategies Discuss why birds need a more efficient way to get oxygen than reptiles. (Muscles require oxygen to extract energy from stored glucose; there is little oxygen at high altitudes.) Have students compare the movements of reptiles and birds. Ask: What do muscles need to keep working efficiently? (Oxygen) Expected Outcome Balloons are more quickly inflated with a hand pump than by mouth. Analyze and Conclude 1. Using the pump is faster. Mouth inflation requires more effort. 2. Mouth inflation simulates the flow of air in and out of the same opening, as in reptile lungs. Air flows in one end of the pump and out the other, as in bird lungs. 3. Flight requires a high level of cellular respiration in the muscles. Also, high-altitude flight requires extra oxygen. Therefore, birds need a large oxygen supply. Birds sing to communicate Birds use sound to communicate with one another. Ornithologists differentiate between two kinds of bird sounds. Calls are short sounds used to warn others of danger and to communicate between members of the same species. Most calls are innate. Songs are longer vocalizations that involve many different notes. Many bird songs involve learning. Some songbirds even learn new songs each year. Birds raised in soundproof environments do not learn songs. In most cases, only males learn songs to establish and defend territories and to attract mates. As in humans, vibrating membranes in birds produce sound. These membranes are located in the syrinx at the posterior end of the trachea. Muscles in the syrinx cause the different pitches in the calls and songs. Answers to... It maintains a high metabolic rate and supplies the extra oxygen required for high-altitude flight. Figure 31 15 Air travels in one direction only, rather than in and out. Figure 31 16 To maintain high levels of oxygen in muscles Figure 31 17 To control precise movements and complex behaviors Reptiles and Birds 811
31 2 (continued) Make Connections Physics Challenge student groups to experiment with different wing shapes to produce flight. Give groups toy gliders made from foam or balsa, or have them construct gliders from paper. Encourage students to change the shape of the wings and nose to improve lift. You might need to explain how air moving over the wings and the nose of the plane provides lift, the upward force. Have groups race their planes to see which fly fastest and which fly farthest. Address Misconceptions Some students might think that birds simply move their wings up and down during flight. Explain that birds move their wings in a circular motion, similar to the movement of oars when rowing. On the downstroke, feathers are held together, and the wings move down and rotate forward. This motion pushes air down and back, providing lift and propelling the bird through the air. On the upstroke, feathers are opened up to allow air to move through them, making it easier to move the wings upward. The wings bend up, moving closer to the body, and rotate backward. Groups of Birds Classifying Have students devise their own criteria for grouping the bird orders pictured in Figure 31 19. You might also give students pictures of birds from other orders that are not pictured in the figure. For their classification systems, students should describe what features they are using to define the characteristics of each order. Pelvic girdle Tailbone Strut Air space Figure 31 18 Like most of its anatomy, a bird s skeleton is well adapted for flight, providing a sturdy attachment point for muscles. The long bones are exceptionally strong and light because of cross-bracing and air spaces. In strong flying birds, such as pigeons, the chest muscles may account for as much as 30 percent of the animal s mass. Calculating If this pigeon has a mass of 200 grams, and 30 percent of its mass is chest muscles, what is the mass of its chest muscles? Vertebra Sternum Rib cage Skull Collarbone (wishbone) Pectoral girdle Movement Some birds cannot fly. Instead, they get around mainly by walking or running, like ostriches, or by swimming, like penguins. However, the vast majority of birds can fly. The skeletal and muscular systems of flying birds exhibit adaptations that enable flight. Observe a bird s skeletal system in Figure 31 18. Although the bones in a bird s wings are homologous to the bones in the front limbs of other vertebrates, they have very different shapes and structures. In flying birds, many large bones, such as the collarbone, are fused together, making a bird s skeleton more rigid than a reptile s. These bones form a sturdy frame that anchors the muscles used for flight. The bones are strengthened by internal struts similar to those used in the framework of tall buildings and bridges. Air spaces make many bones lightweight. Birds also have large chest muscles that power the upward and downward wing strokes necessary for flight. The muscles attach to a long keel that runs down the front of an enlarged breastbone, or sternum. Reproduction In birds, both male and female reproductive tracts open into the cloaca. The sex organs often shrink in size when the birds are not breeding. As birds prepare to mate, the ovaries and testes grow larger until they reach functioning size. Mating birds press their cloacas together to transfer sperm from the male to the female. Some male birds have a penis that transfers sperm to the female s cloaca. Bird eggs are amniotic eggs. They are similar to the eggs of reptiles but have hard outer shells. Most birds incubate their eggs until the eggs hatch. When a chick is ready to hatch, it uses a small tooth on its bill to make a hole in the shell. After much pushing, poking, and prodding by the chick, the eggshell breaks open. Once the exhausted bird has hatched, it collapses for a while and allows its feathers to dry. Both parents may be kept busy providing food for their hungry offspring. Do birds have external or internal fertilization? Groups of Birds Birds fill the woods and fields with song. Imagine how dull the world would be without the color, song, and variety of birds. With nearly 30 different orders, it is impossible to present each type of bird here. Instead, Figure 31 19 provides an overview of some better-known groups and their adaptations. By far, the largest order of birds is the passerines (pas-uh-reenz), or perching birds. This group includes songbirds such as larks, sparrows, and finches. There are over 5000 species of perching birds. Air pressure lifts birds Birds are able to fly because of lift. Lift is the difference in air pressure above the wing and below the wing. As birds fly, air moves across both sides of the wings. Air pressure below the wing is greater than air pressure above the wing. In effect, the air is lifting the wing. Moving air exerts less pressure than air that is not moving. The faster air moves, the less pressure it exerts. Bird wings are shaped so that air moving across the top of the wings is moving faster. The top of a bird s wing is rounded, forcing the air to travel a greater distance in the same amount of time as the air moving under the wing. Because the air moving across the top of the wing has to go a greater distance in the same time span, it must move faster. 812 Chapter 31
FIGURE 31 19 DIVERSITY IN BIRDS Birds show remarkable diversity and inhabit many different environments. Some representative groups are shown below. Applying Concepts Which group of birds is the largest? PARROTS Colorful and noisy, these birds use their feet to hold food. Examples: macaws, lovebirds, cockatoos. PELICANS AND THEIR RELATIVES These birds are found in all types of aquatic ecosystems, from open ocean to lakes and rivers. All have four toes that are connected by a web. Examples: pelicans, cormorants, boobies, frigatebirds (shown here). BIRDS OF PREY These birds, also known as raptors, are fierce predators with hooked bills, large wingspans, and sharp talons. Examples: condors, hawks, owls, eagles (shown above), falcons. Use Visuals Figure 31 19 Encourage students to compare and contrast the features of birds from different orders. Ask: How are parrots different from perching birds? (Parrots use their feet to hold up food.) What features are common to birds of prey? (Hooked bills, large wingspans, and sharp talons) How are pelicans and herons similar? (Both live in aquatic habitats.) PERCHING BIRDS Also called passerines, this is by far the largest order of birds. Many are songbirds. Examples: sparrows, crows, mockingbirds, cardinals (shown here). CAVITY-NESTING BIRDS These multicolored birds live in holes that they make in trees, mounds, or underground tunnels. Examples: barbet (shown here), toucans, woodpeckers. HERONS AND THEIR RELATIVES These birds are adapted to wading in a variety of aquatic habitats. Examples: storks, ibises, spoonbills, herons (shown here), cranes. OSTRICHES AND THEIR RELATIVES These flightless birds must move by running or swimming. Examples: ostriches (shown here), rheas, emus, cassowaries, kiwis. Classifying birds Scientists group birds into more than 20 orders. To classify birds, scientists use characteristics such as bills, wings, tails, and feet. These specialized characteristics enable birds to inhabit different types of environments, move in different manners, or get different types of food. Birds with similar specialized characteristics often inhabit similar environments or have similar niches. Peacocks, for example, belong to the order Galliformes. These ground-dwelling birds have short, stout bills. Their heavy feet have short, strong claws adapted for running and scratching the ground. They have short wings and are poor fliers. Herons, as well as flamingoes and storks belong to the order Ciconiiformes. These wading birds have long legs and necks and broad feet that are not usually webbed. Answers to... Birds have internal fertilization. Figure 31 18 60 grams Figure 31 19 Perching birds (passerines) Reptiles and Birds 813
31 2 (continued) Ecology of Birds Make Connections Environmental Science Have interested students read Rachel Carson s book Silent Spring. Then, invite the students to lead a class discussion about the book in which they explain why they think Ms. Carson wrote the book and how effective her message was. Point out that at the time, the Department of Agriculture was advocating the use of many different types of dangerous chemicals to combat insect pests. Discuss whether or not students think that Ms. Carson s book is relevant today. 3 ASSESS Evaluate Understanding Call on students at random to describe the characteristics of birds. Then, call on other students to describe ways in which birds are adapted for flight. Figure 31 20 This hummingbird uses its long, thin beak to draw nectar from a flower. While feeding, the bird may pick up pollen on its beak and carry it to the next flower it visits, thereby helping the flower to pollinate. Applying Concepts Which type of ecological relationship is represented by the hummingbird and the flower: parasitism, mutualism, or commensalism? Ecology of Birds Because birds are so numerous and diverse, they interact with natural ecosystems and human society in many different ways. For example, hummingbirds, like the one in Figure 31 20, pollinate flowers in both tropical and temperate zones. Fruiteating birds swallow seeds but may not digest them, so their droppings disperse seeds over great distances. Insect-eating birds, such as swallows and chimney swifts, catch great numbers of mosquitoes and other insects, and therefore help control insect populations. Many birds migrate long distances often over hundreds of kilometers of open sea. Such migrations are usually seasonal. It can be startling during a winter visit to a tropical country to see Northern orioles or bright red cardinals flitting around banana trees with parrots and toucans! How do migrating birds find their way? Some species use stars and other celestial bodies as guides. Other species may use a combination of landmarks and cues from Earth s magnetic field. Because birds are highly visible and are an important part of the biosphere, they can serve as indicators of environmental health. It is no accident that conservationist Rachel Carson chose songbirds for the focus of her pioneering campaign in the 1960s against the careless use of DDT and other pesticides. In her book Silent Spring, Carson described to the public for the first time how pesticides that stay in the environment can accumulate in food chains and cause harm to animals they were never intended to affect. Thanks to the efforts of Carson and other conservationists, many birds especially predators such as eagles and ospreys have returned from the brink of extinction. Reteach Have students use Figures 31 11, 31 14, and 31 19 to review the characteristics of birds and the adaptations for flight. The hearts of both reptiles and birds have two atria. However, most reptiles have a threechambered heart in which the single ventricle is partially divided. Birds have a four-chambered heart in which complete division of the ventricle prevents oxygen-rich blood from mixing with oxygenpoor blood. Some students might also mention that crocodiles and alligators have a four-chambered heart similar to that of birds. 31 2 Section Assessment 1. Key Concept Describe the characteristics of a bird. 2. Key Concept List three ways in which birds are well adapted for flight. 3. What is the possible evolutionary relationship between birds and dinosaurs? 4. How does a chick get out of its eggshell? 31 2 Section Assessment 5. Critical Thinking Applying Concepts Crops and gizzards are especially common and well developed in seed-eating birds but less common in carnivorous birds. Explain why crops and gizzards are more advantageous to seed-eating birds than to birds that eat meat. Comparing and Contrasting Write a paragraph in which you compare and contrast the structure and function of the hearts of reptiles and birds. Hint: When you compare and contrast two items, it is not enough to describe each one separately. You need to explain how they are similar and how they are different. To help with this task, you might construct a Venn diagram or a compare-and-contrast table. If your class subscribes to the itext, use it to review the Key Concepts in Section 31 2. Answer to... Figure 31 20 Mutualism 814 Chapter 31 1. Reptilelike animal, endothermic, hollow bones, feathers, two legs, wings 2. Highly efficient respiratory, digestive, and circulatory systems; aerodynamic feathers and wings; strong chest muscles; strong, lightweight skeleton 3. Birds descended either directly from dinosaurs or from a common ancestor of modern birds and dinosaurs. 4. The chick uses a small tooth on its bill to poke a hole in the shell. 5. Seeds, with their tough outer coverings, are much more difficult to digest than meat. Crops and gizzards are not advantageous to carnivorous birds, because animal tissue does not require extra softening and grinding to digest.