44 Birds and Mammals hy don’t cats and dogs lay eggs? Why don’t ostriches Wchew their food before they swallow it? Are the zebras and flamingos in the picture alike in any way? In this chapter, you will find the answers to these questions. Also pre- sented in this chapter are the unique characteristics of birds and mammals. You will learn how birds and mam- mals are adapted to reproduce and live in many different environments. What do you think? Science Journal Look at the picture below with a classmate. Discuss what this might be or what is happening. Here’s a hint: It has found a protected place to eat and grow. Write your answer or best guess in your Science Journal. 104 ◆ C ou may have observed a variety of animals in your EXPLORE Yneighborhood. Maybe you have watched birds at a bird feeder. Birds don’t chew their food because they ACTIVITY don’t have teeth. Instead, many birds swallow small pebbles, bits of eggshells, and other hard materials that go into the gizzard—a muscular digestive organ. Inside the gizzard, they help grind up the seeds. Do the activity below as a model of a gizzard in action. Model how a bird’s gizzard works 1. Place some cracked corn, sunflower seeds, nuts or other seeds, and some gravel in an old sock. 2. Roll the sock on a hard surface and tightly squeeze it. 3. Describe the appearance of the seeds after rolling. Observe Describe in your Science Journal how a bird’s gizzard helps it digest food. FOLDABLES ReadingReading &Study& Study Making a Compare and Contrast Study Fold As you study birds SkillsSkills and mammals, use the following Foldable to help you compare and contrast their behavior. 1. Place a sheet of paper in front of you so that the short side is at the top. Fold in the left side and then the right side to divide the paper into thirds. Unfold the paper. 2. Fold the paper into thirds from top to bottom. Then fold it from the top to bottom again and unfold. 3. Trace over all the fold lines and label the table you created with Birds and Mammals at the top, as shown. In the column on the left list: Vertebrate/Invertebrate, Diet, Movement, Body Symmetry, and Young, as shown. 4. Before you read the chapter, write what you know about birds and mammals. As you read the chapter, add informa- tion to the table. C ◆ 105 SECTION Birds Bird Characteristics Birds are versatile animals. Geese have been observed flying ■ Identify the characteristics of at an altitude of 9,000 m, and penguins have been seen under- birds. water at a depth of 543 m. An ostrich might weigh 155,000 g, ■ Identify the adaptations birds while a hummingbird might weigh only 2 g. Some birds can live have for flight. in the tropics and others can live in polar regions. Their diets ■ Explain how birds reproduce and develop. vary and include meat, fish, insects, fruit, seeds, and nectar. Birds have feathers and scales and they lay eggs. Which of these Vocabulary characteristics is unique to birds? contour feather down feather endotherm Bird Eggs Birds lay amniotic (am nee AH tik) eggs with hard preening shells, as shown in Figure 1A. This type of egg provides a moist, protective environment for the developing embryo. The hard shell is made of calcium carbonate, the same chemical that Most birds demonstrate structural makes up seashells, limestone, and marble. The egg is fertilized and behavioral adaptations for flight. internally before the shell forms around it. The female bird lays one or more eggs usually in some type of nest, as shown in Figure 1B. A group of eggs is called a clutch. One or both par- ents may keep the eggs warm, or incubate them, until they hatch. The length of time for incubation varies from species to Figure 1 species. The young are cared for by one or both parents. A nest’s materials, shape, and location are different for different species of birds. For example, this robin’s round nest is built of grasses and mud in a tree. Amniotic fluid Yolk sac Shell Embryo 106 ◆ C CHAPTER 4 Figure 2 The hollow bones of birds are an adaptation for flight. What advantages do the thin cross braces provide? Sternum The sternum has a structure called a keel, which is where flight muscles attach. Tail A bird does not have a bony tail. Leg bone Hollow leg bone Flight Adaptations People have always been fascinated by the ability of birds to fly. Flight in birds is made possible by their lightweight but strong skeleton, wings, feathers, strong flight muscles, and an efficient respiratory system. Well-developed senses, especially eyesight, and tremendous amounts of energy also are needed for flight. Hollow Bones One adaptation that birds have for flight is an Many theories have been internal skeleton, as shown in Figure 2. Many bones of a bird proposed about how birds often are joined together. This provides more strength and more navigate at night. Some scientists hypothesize that stability for flight. Most bones of birds that fly are almost hol- star positions help night- low. These bones have thin cross braces inside that strengthen flying birds find their way. the bones without adding much weight. The hollow spaces Research the location of inside of the bones are filled with air. the North Star. In your Science Journal, infer how What features strengthen a bird’s bones? the North Star might help birds fly at night. A large sternum, or breastbone, supports the powerful chest muscles needed for flight. The last bones of the spine support the tail feathers, which play an important part in steering and balanc- ing during flight and landing. SECTION 1 Birds C ◆ 107 Feathers Birds are the only animals that have feathers. Their bodies are covered with two main types of feathers—contour feathers and down feathers. Strong, lightweight contour feath- ers give a bird its coloring and smooth shape. These are also the Modeling Feather feathers that a bird uses when flying. The contour feathers on Preening the wings and tail help the bird steer and keep it from spinning Procedure out of control. 1. Cut two 15-cm ϫ 15-cm Have you ever wondered how ducks can swim in a pond on pieces of cotton cloth. a freezing cold day and keep warm? Soft, fluffy down feathers 2. Apply petroleum jelly to provide an insulating layer next to the skin of adult birds and one piece of the cloth. 3. Wet both pieces of the cloth cover the bodies of young birds. Birds are endotherms, meaning with water. they maintain a constant body temperature. Feathers help birds maintain their body temperature, and grow in much the same Analysis 1. Compare the two pieces way as your hair grows. Each feather grows from a microscopic of cloth after they have pit in the skin called a follicle (FAHL ih kul). When a feather been wet. In your Science falls out, a new one grows in its place. As shown in Figure 3, the Journal, describe what you shaft of a feather has many branches called barbs. Each barb has observe. Wash your hands. many branches called barbules that give the feather strength. 2. Infer why birds do not have to find shelter from the rain. Why are some young birds covered with down feathers? A bird has an oil gland located just above the base of its tail. Using its bill or beak, a bird rubs oil from the gland over its feathers in a process called preening. The oil conditions the feathers and helps make them water-repellent. Figure 3 Down feathers help keep birds warm. Contour feathers are the feathers used for flight. Contour feather Down feather Barb Shaft Shaft Barbule 108 ◆ C CHAPTER 4 Birds and Mammals Figure 4 Wings provide an upward force called lift for birds and airplanes. Lift Bald eagles are able to soar for long periods of This glider gets lift from its wings the same way time because their wings have a large surface area to a bald eagle gets lift. provide lift. Wings Although not all birds fly, most wings are adapted for flight. Wings are attached to powerful chest muscles. By flapping its wings, a bird attains thrust to go forward and lift to stay in the air. Its wings move up and down, as well as back and forth. The shape of a bird’s wings helps it fly. The wings are curved on top and flat or slightly curved on the bottom. Humans Some birds have become pests in urban areas. copied this shape to make airplane wings, as shown in Figure 4. Research to learn what When a bird flies, air moves more slowly across the bottom than birds are considered pests across the top of its wings. Slow-moving air has greater pressure in urban areas, what effect than fast-moving air, resulting in an upward push called lift. The they have on the urban amount of lift depends on the total surface area of the wing, the environment, and what speed at which air moves over the wing, and the angle of the measures are taken to reduce the problems they wing to the moving air. Once birds with large wings, such as create. Build a bulletin vultures, reach high altitudes, they can soar and glide for a long board showing your results. time without having to beat their wings.