Flap Your Wings by P.D. Eastman

A case of mistaken identity results in Mr. and Mrs. Bird having to contend with a strange egg in their nest. However, not only do they not try to get rid of the egg, but they look after it as if it were their very own — never mind that it is so big that both of them can sit on it at the same time! And even when the egg hatches and the baby looks nothing like them — or like any bird, for that matter — they try their darnedest to feed and nurture it until the time comes for it to fly (or attempt to, anyway) the roost.

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