“Bluebirds and Wrens and Martins like to live in holes and boxes, though they can’t make holes for themselves,” said Rap.

“Yes, the habits of many birds have changed since the country has become civilized and House People are to be found in all parts of it.  Many birds, who have always been favorites with man, and have been protected by him, have gradually grown less wild, or almost tame, and now prefer living near houses and barns to building in wilder places.  The Bluebird, Martin, and Wren are three very popular birds.  They appreciate cosy homes and are grateful for the boxes built for them, though we know that before they had such things they must have nested in tree holes.”  “I wonder where the Chimney Swifts lived before there were any chimneys,” said Rap, looking across the fields to where an old stone chimney stood—­the only thing left standing of an old farmhouse.  Above this chimney, Swifts were circling in shifting curves, now diving inside it, now disappearing afar in the air.

“We think they must have lived in hollow trees as the Tree Swallows do now,” said the Doctor; “but when House People began to clear the land they naturally cut down the dead trees first, and so the birds moved to the chimneys.”

“I used to call those birds Chimney ‘Swallows,’ but Olive says they are made more like Hummingbirds and Nighthawks than real Swallows,” continued Rap.

“Nighthawks?” said Nat.  “I thought Olive said Hawks were cannibal birds.  How are they relations of Swallows?”

“That is a mistake a great many people make,” said the Doctor; “for the Nighthawk is not a real Hawk, but a shy bird, who has a rapid hawk-like flight, though it eats nothing but beetles, moths, and other insects.  Hark!  Do you hear that cry high in the air?”

“As if something was saying ’shirk-shirk’?” said Nat.

“Yes; that is a Nighthawk on its way home.  Look! he is over us now, and you can see two large white spots like holes in his wings.  By these you can tell it from any of the real Hawks.”

“Does he build high up in a tree?” asked Rap.  “I have never found his nest.”

“There is a good reason for that,” said the Doctor.  “There is no nest.  Two eggs are laid on the bare ground, that is about the same color as the bird itself; and the eggs look too much like streaky pebbles to be easily seen.  When the young are hatched they keep still until they are able to fly, and are colored so exactly like the place upon which they rest that it is almost impossible to see them, even if you know where they are.”

“How much there is to learn!” sighed Nat.  “I’m afraid you will have to make us a big book instead of a little one, Uncle Roy, to teach us all these things.  Olive and Rap have such a start of us.  Dodo and I don’t know much of anything, and even what I thought I knew about birds isn’t very true.”