weather very much?

"He does not stay in the United States until the weather is cold enough
to dull him; but he has to move away for another reason. The same reason
that forces so many birds to leave us--he must follow his food. This
food consists of insects--different kinds of flies, ants, and
grasshoppers, which disappear or die as the air grows cold.

"Rap, have you ever noticed the difference between the sounds in a
spring night and a night in autumn? In spring the air is humming with
the calls of all sorts of insects, but in autumn it is silent, and even
the crickets have stopped chirping.

"So about the last of September our Kingbirds, who live everywhere in
the United States, gather in flocks, start to find a place where insects
are still stirring about, and fly southward, following the sea-coast and
the great rivers for paths. Those from the eastern part of the country
stop in Central America or fly on to South America, and those from the
western part often stop in Mexico."

"But how can they fly so far?" said Nat; "it's hundreds of miles; and
how do they find the way?"

"The flight of a bird is a wonderful thing, my boy. He spreads those
frail wings of his, and launches into the air, up, up, above trees and
steeples, then on and on, being able to fly several hundred miles
without resting. Some birds, when the wind aids them, cover more than a
hundred miles in a single hour.

"As to the way, the eye of the bird is like a telescope. It magnifies