If swallows studied history, 1920 would have been an important date for Eve and Petro. It was the one hundredth anniversary of the year when a man named Long visited cliff swallows among the Rocky Mountains.

The century between 1820 and 1920 had given what we call civilization a chance to make many changes in the wild world of birds. During that time lifeless hummingbirds had been made to perch upon the hats of fashionable women; herring gulls had been robbed of their eggs and killed for their feathers; shooting movements had been organized to kill crows with shotgun or rifle, in order that more gunpowder might be sold; the people of Alaska had been permitted to kill more than eight thousand eagles in the last great breeding-place left to our National Emblem; uncounted millions of Passenger Pigeons had been slaughtered, and these wonderful birds done away with forever; and the methods by which egrets had been murdered were too horrible to write about in books for children to read.

But however shamefully civilization had treated, and had brought up children to treat, these and many other of their fellow creatures of the world, who had a right to the life that had been given them as surely as it had been given to men, the years since 1820 had been happy ones for the ancestors of Eve and Petro.

Eve and Petro, themselves, were happy as any two swallows need be that spring of 1920, when they started forth to seek a cliff, just as their ancestors had done for the hundred years or so since man began to notice their habits, and no man knows for how many hundreds of years before that.

Of course they found it as all cliff swallows must, for cliff-hunting is a part of their springtime work. It was very high and very straight. Its wall was of boards, and the gray shingled roof jutted out overhead just as if inviting Eve and Petro to its shelter.

It was a good cliff, and mankind had been so busy building the same sort all across the country for the past hundred years that there was no lack of them anywhere, and swallows could now choose the ones that pleased them best. Yes, civilization had been kind to them and had made more cliffs than Nature had built for them; though perhaps it was Mother Nature, herself, who taught the birds that these structures men called barns and used inside for hay or cattle were, after all, only cliffs outside, and that people were harmless creatures who would not hurt the swallow kind.

However all that may be, it is quite certain that Eve and Petro squeaked pleasantly for joy when they chose their building site, undisturbed by the ladder that was soon put near, and unafraid of the people who climbed up to watch them at their work. They were too happily busy to worry, and besides, there is a tradition that men folk and swallow folk are friendly, each to the other.

How old this tradition is, we do not know; but we do know that swallows of one kind and another were welcomed in the Old World in the old days to heathen temples before there were Christian churches, and that to-day in the New World they play in and out of the dark arches in the great churches of far Brazil and flash across the gilding of the very tabernacle, reminding us of the passage in the Psalms where it is written that the swallow hath found a nest for herself, where she may lay her young—even thine altars, O Lord of Hosts!

So it is not strange that far and wide over the world people have the idea that swallows bring luck to the house. I think so myself, don't you?—that it is very good fortune, indeed, to have these birds of friendly and confiding ways beneath our shelter.

Of course the ancestors of cliff swallows had not known the walls and roofs of man so long as other kinds of swallows; but the associations of one short century had been pleasant enough to call forth many cheerful squeakings of joy, just like those of Eve and Petro that pleasant day in June when they started their nest under the roof near the top of the ladder.

To be sure, they made no use of that ladder, even though they were masons and had their hods of plaster to carry way up near the top of their cliff. No, they needed no firmer ladder than the air, and their long wings were strong enough to climb it with.

They lost little time in beginning, each coming with his first hod of plaster. How? Balanced on their heads as some people carry burdens? No. On their backs, then? No. In their claws? Oh, no, their feet were far too feeble for bearing loads. Do you remember what Corbie used for a berry-pail when he went out to pick fruit? Why, of course! the hod of the swallow mason is none other than his mouth, and it holds as much as half a thimbleful.

First, Eve had to mark the place where the curved edge of the nest would be; and how could she mark it without any chalk, and how could she make a curve without any compasses? Well, she clung to the straight wall with her little feet, which she kept nearly in one place, and, swinging her body about, hitch by hitch, she struck out her curve with her beak and marked it with little dabs of plaster. Then she and Petro could tell where to build and, taking turns, first one and then the other, they began to lay the wall of their home.

It was slow work, for it must be thick and strong, and the place where they gathered the plaster was not handy by, and it took a great great many trips, their hods being so small.

At first, while the nest was shallow, only one could work at a time; and if Petro came back with his plaster before Eve had patted the last of hers into place, she would squeak at him in a fidgety though not fretful voice, as if saying, "Now, don't get in my way and bother me, dear." So he would have to fly about while he waited for her to go. The minute she was ready to be off, he would be slipping into her place; and this time she would give him a cosy little squeak of welcome, and he would reply, with his mouth full of plaster, in a quick and friendly way, as if he meant, "I'll build while you fetch more plaster, and we'd both better hurry, don't you think?"

After worrying a bit about the best place to dump his hodful, he went to work. He opened his beak and, in the most matter-of-fact way, pushed out his lump of plaster with his tongue, on top of the nest wall. Then he braced his body firmly in the nest and began to use his trowel, which was his upper beak, pushing the fresh lump all smooth on the inside of the nest.

Have you ever seen a dog poke with the top of his nose, until he got the dirt heaped over a bone which he had buried? Well, that's much the way Petro bunted his plaster smooth—rooted it into place with the top of his closed beak. He got his face dirty doing it, too, even the pretty pale feather crescent moon on his forehead. But that didn't matter. Trowels, if they do useful work, have to get dirty doing it, and Petro didn't stop because of that. If he had, his nest would have been as rough on the inside as it was outside, where a humpy little lump showed for each mouthful of plaster.

Although Eve and Petro did not fly off to the plaster pit together, they did not go alone, for there was a whole colony of swallows building under the eaves of that same barn; and while some of them stayed and plastered, the rest flew forth for a fresh supply.

They knew the place, every one of them; and swiftly over the meadow and over the marsh they flew, until they came to a pasture. There, near a spring where the cows had trampled the ground until it was oozy and the water stood in tiny pools in their hoof prints, the swallows stopped. They put down their beaks into the mud and gathered it in their mouths; and all the time they held their wings quivering up over their beautiful blue backs, like a flock of butterflies just alighting with their wings atremble.

So their plaster pit was just a mud-puddle. Yes, that is all; only it had to be a particularly sticky kind of mud, which is called clay; for the walls of their homes were a sort of brick something like that the people made in Egypt years and years ago. And do you remember how the story goes that the folk in Pharaoh's day gathered straws to mix with the clay, so that their bricks would be stronger? Well, Eve and Petro didn't know that story, but they gathered fibres of slender roots and dead grass stems with their clay, which doubtless did their brick plaster no harm.

Men brick-makers nowadays bake their bricks in ovens called kilns, which are heated with fire. Eve and Petro let their brick bake, too, and the fire they used was the same one the Egyptians used in the days of Pharaoh—a fire that had never in all that time gone out, but had glowed steadily century after century, baking many bricks for folk and birds. Of course you know what fire that is, for you see it yourself every day that the sun shines.

Every now and again Eve and Petro and all the rest of the swallow colony left off their brick-building and went on a hunting trip. They hunted high in the air and they hunted low over the meadow. They hunted afar off along the stream and they hunted near by in the barnyard. And all the game they caught they captured on the wing, and they ate it fresh at a gulp without pausing in their flight. As they sailed and swirled, they were good to watch, for a swallow's strong long wings bear him right gracefully.

Why did they stop for the hunting flight? Perhaps they were hungry. Perhaps their mouths were tired of being hods for clay they could not eat. Perhaps the fresh plaster on the walls of their homes needed time to dry a bit before more was added.

Be that as it may, they made the minutes count even while they rested from their building work. For they used this time getting their meals; and whenever they were doing that, they were working for the owner of the barn, paying their rent for the house-lot on the wall by catching grass insects over the meadow, and mosquitoes and horseflies and house-flies by the hundreds, and many another pest, too.

Ah, yes, there may be some reason for the belief that swallows bring good luck to men. I once heard of a farmer who said he didn't dare disturb these birds because of a superstition that, if he did, his cows wouldn't give so much milk. Well, maybe they wouldn't if all the flies a colony of swallows could catch were alive to pester his herd; for the happier and more comfortable these animals are, the healthier they are and the more milk they give.

The hunting flights of Eve and Petro and their comrades lasted about fifteen minutes each time they took a recess from their building.

After two days the nest was big enough, so that there was room for both swallows to build at once; and after that, Petro didn't have to fly around with his mouth full of plaster waiting for Eve to go if he chanced to come before she was through. They always chatted a bit and then went on with their work, placing their plaster carefully and bunting it smooth on the inside, modeling with clay a house as well suited to their needs as is the concrete mansion a human architect makes suited to the needs of man.

And if you think it is a simple matter to make a nest of clay, just go to the wisest architect you know and ask him these questions. How many hodfuls of clay, each holding as much as half a thimble, would it take to build the wall of a room just the right shape for a swallow to sit in while she brooded her eggs? How large would it have to be inside, to hold four or five young swallows grown big enough for their first flight? How thick would the walls have to be to make it strong enough? What sort of curve would be best for its support against a perfectly straight wall? How much space would have to be allowed for lining the room, to make it warm and comfortable? How can the clay be handled so that the drying sun and wind will not crack the walls? What is the test for telling whether the clay is sticky enough to hold together? How much of the nest must be stuck to the cliff so that the weight of it will not make it fall?

If the architect can answer all those questions, ask him one more: ask him if he could make such a nest with the same materials the birds used, and with no more tools?

Well, Eve and Petro could and did. It was big enough and strong enough and shaped just right; and when it was nearly done and nearly ready for the soft warm lining, That Boy climbed the ladder and knocked it down with his hand.

There it lay, Eve and Petro's wonderfully modeled nest of clay, broken to bits on the ground and spoiled, oh, quite spoiled. There is a saying that it brings bad luck to do harm to a swallow. What bad luck, then, had the hand of That Boy brought to the world that day?

Bad luck it brought to Eve and Petro, who had toiled patiently and unafraid beside the ladder-top, with faith in those who climbed quietly to watch the little feathered masons at their work. But now the walls of their home were broken and crumbled, and their faith was broken and crumbled, too. In dismay they cried out when they saw what was happening, and in dismay their swallow comrades cried out with them. Fear and disappointment entered their quick hearts, which had been beating in confidence and hope. People who climbed ladders were not beings to trust, after all, but frightful and destroying creatures. This had the hand of That Boy brought to Eve and Petro, who looked at the empty place where their nest had been, and went away.

Bad luck it brought to an artist who drew pictures of birds; and when he knew what had happened, a sudden light flamed in his eyes. The name of this light is anger—the kind that comes when harm has been ruthlessly done to the weak and helpless. For the artist had climbed the ladder many a time, and had laid his quiet hand upon the lower curve of the nest while Eve and Petro went on with their building at the upper edge. And he had seen the colors of their feathers and the shape of the pale crescent on their foreheads—the mark a man named Say had noticed many years before, when he named this swallow in Latin, lunifrons, because luna means moon and frons means front. And he had hoped to climb the ladder many a time again, and when there should be young in the nest, to see how they looked and watch what they did, so that he could draw pictures of the children of Eve and Petro.

Bad luck it brought to a writer of bird stories; and when she knew what had happened, something like an ache in her throat seemed to choke her, something that is called anger—the kind that comes when harm is done to little folk we love. For she had climbed the ladder many a time, and had rested her head against the top while she watched Eve and Petro push the pellets of mud from their mouths with their tongues and bunt the wall of their clay nest smooth on the inside with the top of their closed beaks, not stopping even though they brushed their pretty chestnut-colored cheeks against the sticky mud, or got specks on the feathers of their dainty foreheads that bore a mark shaped like a pale new moon. And she had hoped to climb the ladder many a time again, and watch Eve and Petro feed their children when the nest was done and lined and the eggs were laid and hatched; for this nest could be looked into, as the top was left open because the barn roof sheltered it and it needed no other cover.

Now Eve and Petro were gone, and no more sketches could be made near enough to show how little cliff swallows looked in their nest. And nothing more could be written about such affairs of these two birds as could only be learned close to them. Nor, indeed, was there any way to learn those things from the rest of the colony; for it so chanced that Eve and Petro were the only pair who had built where a ladder could be placed. So bad luck had come not only to Eve and Petro, but to the story of their lives.

But, most of all, the breaking of their nest brought bad luck to That Boy, himself. For as he stood at the top of the ladder, he might have curved the hollow of his hand gently upon the rounded outside of the nest and, waiting quietly, have watched the building birds. He might have seen Eve come flitting home with her tiny load of clay, poking it out of her mouth with her tongue and bunting it smooth in her own cunning way. He might have laid his head against the ladder and heard their cosy voices as they squeaked pleasantly together over the home-building. He might have looked at the colors of their feathers, and seen where they were glossy black with a greenish sheen, where rich purply chestnut, and where grayish white. He might have looked well at the pale feather moon on their foreheads, which the man named Say had noticed one hundred years before. He might, oh, he might have become one of the brotherhood of men, whom swallows of one kind or another have trusted since the far-off years of Bible times when they built at the altars of the Lord of Hosts.

All this good luck he held, That Boy, in the hollow of his hand, and he threw it away when he struck the nest; and it fell, crumbled, with the broken bits of clay.

As for Eve and Petro, if fear and disappointment had driven trust from their hearts, they still had courage and patience and industry. They sought another and a different sort of cliff, and found one made of red brick and white stone. Near the very high top of this a large colony of swallows were building; and, because there was no closely protecting roof, these swallows were making the round part of their nest closed over at the top with a winding hallway to an outer doorway. They looked, indeed, like a row of quaint clay pottery, shaped like crook-necked gourds. For such were the nests these swallows built one hundred years ago on the wild rock cliffs, if they chose their house-lots where there was no overhanging shelter; and such are the nests they still build when there seems to be need of them.

They were too far from the pleasant pasture to dig their clay out of the footprints of cows; but there was a track where the automobiles slushed through sticky mud, and they swirled down there and filled their little hods when the road was clear.

Eve and Petro found a nook even higher up than the others, where a crook-necked jug of a nest did not seem to fit. When they had built their wall as high as need be, they closed it over with a little rounded dome, and at the side they left two doorways open, one facing the southwest and one facing the southeast. And some days after this was done, had you gone to the foot of their cliff and used a pair of field-glasses, you might have seen Eve's head sticking out of one door and Petro's at the other. Ah, they had, then, some good luck left them. They had had each other in their days of trouble, and now they rested from their building labors and sat happily together in their second home, each with a doorway to enjoy.

And later on they had more good luck still. For there came a day when they spent no more time sitting at ease within doors, but flew hither and yon, and then, returning to the nest, clung outside with their tiny feet and stuck their heads in at the open doorway for a brief moment before they were off again. Their nest was too far up for anyone to hear or see what went on within; but there must have been some hungry little mouths yawning all day long, to keep Eve and Petro both so busy hunting the air for insects.

Soon after this one of the doors was closed, sealed tight with clay. What had happened? Were the little ones inside crowding about too recklessly, so that there was danger of one falling out? Had Eve and Petro come upon an especially good mud-puddle and built a bit more just for the fun of it?

It was not very many days after this that Eve and Petro and all their comrades ceased coming to the cliff where their curious nests were fastened. Their doorways knew them no more; but over the meadows from dawn till nearly dusk there flew beautiful old swallows bearing upon their foreheads the pale mark of a new moon, and with them were their young.

At night they sought the marshes, where their little feet might cling to slender stems of bending reeds; and their numbers were very many.

But winter would be coming, and if it still was a long way off, so were the hunting grounds of South America, where they must be flitting away the days when the northern marshes would be frozen over.

So off they went, Eve and Petro and their young, looking so much like others of the swallow flock that we could not tell who they were, now that they had stopped coming to their nest with one open and one closed doorway.

They would have far to travel, even if they took the direct over-water route, which many sorts of birds do. But what is distance to Petro, whose strong wings carry him lightly? A mile or a hundred or a thousand even are nothing if the hunting be good. Might just as well be flying south, as back and forth over the same meadow the livelong day, with now and then a rest on the roadside wires, which fit his little feet nearly as well as the reeds of the marsh. Some people think it is for the sake of the hunting that the route of the swallows lies overland, for they fly by day and catch their game all along the way.

And as they journeyed, Eve and Petro and their flock, south and south and south, maybe the children, here and there, waved their hands to them and called, "Good hunting, little friends of the air, and good luck through all the winter till you come back to us again."