| A | B |
|---|
| feathers | unique to birds, composed mainly of the versatile protein keratin |
| wings | a bird's forelimbs are modified into a pair of wings |
| lightweight rigid skeleton | 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 |
| endothermic metabolism | a bird's rapid metabolism supplies the energy needed for flight, |
| 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 | the jaws are covered by a tought, horny sheath |
| 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 |
| furcula | the fused collarbones commonly called a wishbone |
| down feathers | cover the body of nestling birds and provide an insulating undercoat in adults |
| contour feathers | give adult birds their streamlined shape and provide coloration and additional insulation |
| flight feathers | specialized contour feathers on the wings and tail |
| follicles | feathers develop from tiny pits in the skin |
| shaft | what emerges from the follicle |
| vanes | the part of the feather that develops on both sides of the shaft |
| barbs | the many branches on each vane on the feather |
| barbules | the many projections coming out of the barbs |
| preening | feather care, when birds rub their beaks on thier feathers and oil is secreted |
| preen gland | the gland that secretes preening oil |
| sternum | breastbone, also an attachment point for flight muscles |
| pygostyle | the terminal fused vertebrae of the spine, that supports the tail feathers, which play an important part in flight |
| proventriculus | the first chamber of the stomach where digestive enzymes begin to breakdown food |
| crop | an enlargement of the esophagus |
| gizzard | the muscular portion of the stomach, which kneads and crushes the food |
| vasa deferentia | small tubes which sperm pass through |
| oviduct | the long funnel shaped duct where eggs are released into |
| brood patch | the thickened featherless patch of skin on the abdomen |
| precocial | young birds that are active as soon as they hatch |
| altricial | birds that are blind, naked, and helpless when they hatch |
| Ornithologists | biologists who study birds |
| syrinx | the structure where song is produced in birds which is located at the base of the bird's trachea |
| crop milk | a nutritious milk like fluid used to store food in most other birds |
