“Hush!  Wait a moment and listen to that Thrasher,” said the Doctor, stopping behind some thick bushes; “he is wooing his mate!”

“What is wooing?” whispered Nat.

“Asking her to marry him and come and build a cosy home in one of these nice bushes.  Listen!  See!  There he is, up on the very top of that young birch, with his head thrown back, singing as if his throat would split.”  As the children looked up they saw a fine bird with a curved beak, rusty-brown back, and light breast streaked with black, who was clinging to a slender spray, jerking his long tail while he sang.

“It seems as if I could almost hear the words he says,” said Rap.

“Birds sing in many different tones,” said the Doctor.  “The Thrasher’s song is like some one talking cheerfully; the Meadowlark’s is flute-like; the Oriole’s is more like clarion notes; the Bobolink bubbles over like a babbling brook; while the dear little brown striped Song Sparrow, who is with us in hedge and garden all the year, sings pleasant home-like ballads.”

“There are some birds that Olive told me can’t sing a bit,” said Nat, “but only call and squeak.  How do they ask their mates to marry them?”

“All birds have alarm cries, and a call-note that serves the same purpose as a song, although it may not seem at all musical to us.  We are naturally more interested in that order of birds whose voices are the most perfectly developed.  These not only sing when they are courting, but all the time their mates are sitting upon the eggs, and until the young are ready to fly.”

“Why do birds always build nests in spring?” asked Nat.

“I think because there is more for them to feed the little ones with, than when it gets to be hot and dry,” said Rap, “and it gives them time to grow big and strong before winter comes, when they must go away.”

“Quite right, Rap, and it also gives the parents a chance to shed the old feathers that have been worn by rubbing on the nest, grow a new, thick, warm coat for winter, and rest themselves before they set out on their autumn journey.  Do you remember what I told you that rainy day in my study about this moulting or changing of feathers?”

“Yes, I do,” said Rap and Nat together.  “Most birds have two coats a year, and the male’s is the brighter,” continued Nat eagerly, proud to show that he remembered.  “The one that comes out in the spring is the gayest, so that his mate shall admire him and when this coat comes he sings his very best and—­”

“Stop and take breath, my boy,” laughed the Doctor; “there is plenty of time.  Why do we think that the male has the gayest feathers—­do you remember that also?”

“No, I’ve forgotten,” said Nat.

“I remember,” cried Rap; “it is to please the female and because she sits so much on the nest that if her feathers were as bright as the male’s her enemies would see her quicker, and when the little birds hatch out they are mostly in plain colors too, like their mother.”