feather Structure made mostly of protein that develops from a pit in a bird’s skin.
endotherm Animal that generates its own body heat and controls its body temperature from within.
crop In earthworms, part of the digestive system in which food can be stored; in birds, structure at the lower end of the esophagus in which food is stored and moistened.
gizzard In earthworms, part of the digestive system in which food is ground into smaller pieces; in birds, a muscular organ that helps in the mechanical breakdown of food.
Air sac One of several sacs attached to a bird’s lungs into which air moves when a bird inhales; allows for the one-way flow of air through the respiratory system.
Key Concept Birds are reptile-like 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.
Key Concept 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.