"It is twice as high as the one they built here last summer." The Doctor felt of the bottom of the nest very gently with one finger and said, "I thought so! You have sharp eyes, Rap; it is very thick, and for a good reason--it is a two-storied nest!" "A two-storied nest! Are there such things ?" clamored the children together.
"The mother-bird is worrying; come over under the mulberry tree and I will tell you about this wonderful nest.
"There are some very ill-mannered shiftless Citizens in Birdland, called Cowbirds," began the Doctor; "you will learn about them when we come to the family to which they belong.

They build no nests, but have the habit of laying their eggs in the nests of other birds, just as the equally bad-behaved Cuckoos do in Europe.

Some birds do not seem to know the difference between these strange eggs and their own, and so let them remain until they are hatched.

Others are wise enough to know their own eggs, and chief among such sharp-eyed ones is this little Yellow Warbler.
"Coming home some morning after taking exercise for the good of her health, Mrs.Warbler finds a great white egg spotted with brown, crowded in among her own small pale blue eggs, that have their brown spots mostly arranged like a wreath around the larger end.
"Being disgusted and very angry to find her house invaded, she and her mate have a talk about the matter.

Why they do not simply push the strange egg out, we do not know, but instead of that they often fly off for milkweed fibres and silk to make a new nest right on top of the first one, shutting the hateful egg out of sight underneath.