| A | B |
|---|
| contour feathers | strong, lightweight feathers that give birds their coloring and shape and that are used for flight |
| down feathers | soft, fluffy feathers that provide an insulating layer next to the skin of adult birds and that cover the bodies of young birds |
| preening | process in which a bird rubs oil from an oil gland over its feathers to condition them |
| migration | instinctive seasonal movement of animals to find food or to reproduce in better conditions |
| mammal | endothermic vertebrate that has hair, teeth (specialized for eating) and glands for feeding the young |
| mammary glands | glands of mammals; in females they produce milk to feed the young. |
| monotreme | a mammal that lays eggs with leathery shells and whose mammary glands do not have nipples |
| marsupial | mammals with an external pouch for the development of its immature young |
| placental | mammal whose young develop inside the females's uterus |
| placenta | an organ that develops from tissues of the embryo and tissues that line the inside of the uterus and that absorbs oxygen and food from the mother's blood |
| umbilical cord | connects embryo to placenta; moves food and oxygen from placenta to embryo and removes waste |
| gestation period | period of time during which an embryo develops in the uterus |
