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."

"Don't be discouraged, my boy; you do not need a big book--a little one
will do for the present. What you need is patience, a pair of keen eyes,
and a good memory. With these and a little help from Olive, Rap, and
your old uncle, you can learn to know a hundred kinds of every-day
birds--those that can be found easily, and have eit