The Pinkish, Purplish, Bluish Egg by Bill Peet

Myrtle is a dove whose nest is empty and she's very depressed about it. So, she goes in search for some eggs to sit on so she feels useful. She searches far and wide to find the right kind of eggs to take to her nest and finds a massive one in a cave one day. However, she wonders how she's going to get it back to the nest; until some playful squirrels roll it around and up the tree and put there for her.
It's not long until the other birds and animals notice that pinkish, purplish, bluish egg dominating Myrtle's rather small nest and her sitting on it to keep it warm. And even sooner that the birds and animals voice their opinions of how ridiculous she looks atop of that egg. Even the owl says that egg will never hatch because it's too cold and she's not big enough to keep it warm.
But the egg does hatch; and what comes out of it isn't a bird... in fact, it's far from anything that looks like it.

Bill Peet is famous for working with Walt Disney and writing and illustrating many children's books. There's so much to look at and read on his website that I think I'll let you have a look at it on the sidebar yourself; as I don't really know what to put in here.