Nat and Dodo thought for a moment, and then said they never had.

“It really may not be like a Canary,” said Rap, “and it hasn’t much of a song, but it has so many cute little ways that it seems like one.  I know a boy who always says it’s a wild Canary, but it can’t be that, I see.  A pair of these Warblers have a nest in one of the elder bushes by our fence, and they wouldn’t mind a bit if we went to look at them.  Would it be too far for you to come, sir?” he inquired timidly of the Doctor, evidently proud of having something to show.

[Illustration:  Yellow Warbler]

“We shall be glad to see the nest, my boy.  How is it that you have so many birds about your house?”

“I think it’s partly for the same reason that you have birds here—­for we don’t keep cats either—­and it’s partly because we have four big old mulberry trees.”

“What have mulberry trees to do with birds?” asked Nat, without stopping to think.

“Everything,” said the Doctor.  “The mulberry is one of the most attractive fruits to our familiar birds, and at least twenty-five species feed upon it greedily.

“Whoever plants a mulberry tree in his garden sends a public invitation through Birdland for its people to come and live with him.  The invitation is always accepted, and the birds appreciate the kindness so much that when they find mulberries they leave the cherries and strawberries in that garden in peace.  This should teach us to plant wild fruits and berries for the birds, who prefer them to garden fruits.”

As the children turned from the road into Rap’s garden they saw that it held a great many birds.  The bushes and trees were all untrimmed, and the old house with its shingled sides and coast-backed roof was covered with a trumpet-creeper and some grape vines.

“What a lovely place for Hummingbirds!” cried Olive.

“And Martins,” added the Doctor, pointing to a bird-box with ten or twelve divisions in it, that was fastened under the eaves.

“The Warbler’s nest is here,” said Rap, leading the way to a back fence and feeling very proud at the admiration his home was receiving.

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.”