"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