“There is something else in the nest-lining that looks like feathers,” said Nat.

“That is dandelion down.”

“Don’t you think, Doctor, that this nest is very thick underneath?” asked Rap.  “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.  Then they begin housekeeping anew, in a two-storied nest like this one, living in the upper story, and keeping the Cowbird’s egg locked up in the basement, where no warmth from their bodies can reach it; and so it never hatches.  If a second Cowbird’s egg is laid, in the new upper story of the nest, the Warblers generally abandon their home in despair, and choose a new nesting place; but sometimes they build a third story over the other two, and thus defeat the evil designs of both their enemies without giving up their home.

“This nest of Rap’s is a two-storied one, and when I touched the bottom I could feel that there was an egg in the lower story.  By and by, when the birds have flown, we will take the nest apart and you can see for yourselves how ingeniously it is made.”

“To think of all the ways birds have,” said Rap; “going to such a heap of trouble for something they could fix with one good push.”

“What happens when the Cowbird’s egg stays in the nest and hatches out?  Aren’t the other little birds squeezed and uncomfortable?” asked Dodo.