to keep it burning, only instead of wood or coal we need food. Very early in the morning this life flame of ours, that is called vitality, is very low, like a fire that has burned down, and if we go out in the damp air and breathe the mists that rise from the ground our vitality has not strength to resist them. But if we put fresh fuel on our inward fire by eating something before we go out, then that bad little mischief-maker, which we call malaria, has harder work to creep into us."
"How funny! May I call Rap to tell him? Rap! Rap! come in and have milk and something to eat, to make your inside fire burn up chills and fever!"
Rap thought at first that Nat must be crazy, but very soon understood what the Doctor meant, and was overjoyed at the prospect of having him join the expedition.
"Dodo will cry when she wakes up and knows where we have gone," said Nat, who had been much more kind and thoughtful of his sister since coming to the Farm. But kindness is very catching, and at the Farm everybody was kind, from the House People to the big gray horses in the barn, which let the chickens pick up oats from between their powerful hoofs, without ever frightening them by moving.
"It is too long a walk for little sister, but you must remember everything that you see and hear, and tell her about it. Don't forget the field-glass," said the Doctor, following the boys along the road where telegraph wires made bird-perches between the high poles.
"You said a lot of birds came last night," said Nat to Rap; "but how do you know that they came last night and where did they come from?"
"I know they came last night because they were not here yesterday," answered Rap; "but I don't know where they came from, except that it must be from where it is warmer than it is here, because they went away just before it grew cold last fall. See, Doctor, there are some of them now on those fence rails and more up on the telegraph wires. The miller calls them 'Bee Martins,' and says that they eat up all the honey-bees. Have they any other name—because I have never seen them catch bees?"
Nat looked at them first with the field-glass, then without it as they drew quite near the fence, and saw a fine bird, twice as long as his middle finger. Its back and wings looked dark gray; it was white underneath, with a touch of gray on the breast, and had a black tail, with white at the end of it. As Nat looked the bird raised a little tuft of feathers on top of its head, as if angry, flew into the air, giving a shrill cry, seized an insect, and returned to its perch.
"That is the Kingbird," said the Doctor; "one of the most useful of the insect-catchers. Instead of living on honey-bees, as many people think, he eats very few of these, but kills instead thousands of the bad robber-fly, which is the honey-bee's worst enemy. This bird is really king of the air and of all fly-catching birds. See how graceful his flight is, and how easily be moves!"
"Why did he go away last fall?" asked Nat. "Does he feel the cold 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 and sees from very far off. Flying through the upper air the bird watches the line of coast and river, and the instinct that is placed in him says, 'Follow these.' So he follows them, remembering that by doing so he has found a place of safety in other seasons. All through the spring and all through the autumn birds take these mysterious flights—for so they always seem to House People, as flock after flock gathers and disappears. You can watch them sometimes passing by day so high in the sky that they seem like dust-motes—then perhaps you will only hear a faint call-note and see nothing. At night the sound of many voices falls from the clouds. Sometimes it will be the tinkling bell of Bobolinks, sometimes the feeble peep of Snipes, and sometimes the hoarse honk of Wild Geese."
"Why, Uncle Roy! Can you tell a bird's name without seeing it, only by one little cry?"
"Yes, my lad. When you have lived with birds as long as I have, you will know their different voices as you do those of your own family. When some one calls you in the garden, can't you tell whether it is Dodo or Olive?"
"Yes, but their voices are so very different."
"So are the voices of birds, when you know them well."
"But the young birds who have been hatched up here—how do they know about going the first time?" asked Rap.
"The young ones are led in their journeys with signals and cries by their parents; they in turn lead their own young, and so the knowledge is kept up endlessly."
"I can see why they go south," said Rap, after thinking a few moments, "but why do they come back again? Why don't they stay and build their nests down there?"
"That is a difficult question to answer," said the Doctor, "and one that we House People try to explain in different ways. I think that the love of the place where they were born is strong enough in birds to bring them back every season to build their nests. So you see that Citizen Bird is a patriot; for, though he may be in the midst of plenty in a tropical forest, when the time comes he travels hundreds of miles to his native land to make the young, that will fly from his nest, citizens like himself."
"But the birds that can eat seeds and other things do not travel so far, do they?" asked Rap.
"No, the birds who rove about the United States throughout the year are either Weed Warriors, or Seed Sowers, or those Tree Trappers who creep about tree-trunks picking the eggs and grubs of insects from the bark. Or else those great Cannibal Birds, the Wise Watchers, who eat the flesh of their smaller brothers, as well as of rats, mice, and all such vermin—the Hawks and Owls; or else they are Gulls, Terns, Fishing Ducks, and a great many other kinds of sea birds who feed on fish and pick up the scraps floating on the surface of sea, lake, and river."
"Do the Barn Swallows that are making nests in the hayloft go as far south as Kingbirds?" asked Nat.
"Yes, indeed! The Swallows' swift flight carries them far and wide, for not only do they make homes all through North America, but they are so sure of wing and confident of outstripping any cannibal birds who might try to chase them, that when they leave us they fly by day and often stop for a little visit in the West Indies on their way to South America."
"Suppose, Uncle Roy, when they are travelling, a storm comes up and it grows so foggy they can't see how to follow the rivers—don't they sometimes lose themselves?"
