The children tiptoed up and each took a peep into the cup-shaped nest. The little gold and olive mother, trusting Rap from past experience, gave a quick flip of her wings, and perched on a wild blackberry bush near by. The outside of the nest looked as if it were made of silvery-gray linen floss. There were some horsehairs woven in the lining, and here and there something that 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.