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A muscular, thick-walled part of a bird's stomach that squeezes and grinds partially digested food.
The difference in pressure between the upper and lower surfaces of a bird's wings that produces an upward force that causes the bird to rise.
An endothermic vertebrate with a four-chambered heart and skin covered with fur or hair, that feeds its young with milk from the mother's body.
A mammal whose young are born at an early stage of development, and which usually continue to develop in a pouch on their mother's body.
A mammal that develops inside its mother's body until its body systems can function independently.
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