Although there are many kinds of birds, birds are so distinctive that it is difficult to mistake one for any other kind of vertebrate. All birds—even those that cannot fly—share the seven important characteristics described below.
• Feathers—Feathers are unique to birds, and all birds have them. Like hair, feathers are composed mainly of the versatile protein keratin. Feathers are essential for flight, and they insulate a bird's body against heat loss.
• Wings—A bird's forelimbs are modified into a pair of wings. Feathers cover most of the surface area of the wing.
• Lightweight, rigid skeleton—The skeleton of a bird reflects the requirements of flight. Many of the bones are thin-walled and hollow, making them lighter than the bones of nonflying animals. Air sacs from the respiratory system penetrate some of the bones. Because many bones are fused, the skeleton is rigid and can resist the forces produced by the strong flight muscles.
• Endothermic metabolism—A bird's rapid metabolism supplies the energy needed for flight. Birds maintain a high body temperature of 40-41°C (104-106°F). The body temperature of humans, by contrast, is about 37°C, or 98.6°F.
• Unique respiratory system—A rapid metabolism requires an abundant supply of oxygen, and birds have the most efficient respiratory system of any terrestrial vertebrates. The lungs are connected to several sets of air sacs, an arrangement that ensures that oxygen-rich air is always in the lungs.
• Beak—No modern bird has teeth, but the jaws are covered by a tough, horny sheath called a beak.
• Oviparity—All birds lay amniotic eggs encased in a hard, calcium-containing shell. In most species, the eggs are incubated in a nest by one or both parents.
• Identify and describe seven major characteristics of birds.
• List three similarities between birds and dinosaurs.
• Describe the characteristics of Archaeopteryx.
• Summarize the two main hypotheses for the evolution of flight.
This phylogenetic diagram represents a hypothesis for the relationship among birds, reptiles, mammals, and some extinct relatives of birds. For updates on phylogenetic information, visit go.hrw.com. Enter the keyword HM6 Phylo.
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