t looked like sponge peeped out
between the strands which held the nest firmly in the crotch of the
elder stem.

"What is that soft stuff?" whispered Dodo.

"It is wool scraped from the stalks of young ferns," said the Doctor;
"the soft brown wool that is wrapped round the leaves to keep them warm
in their winter sleep until they stretch out of the ground and feel the
warmth of the sun. The little Warblers gather it in their beaks and mat
it into a sort of felt."

"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 fr