But, speaking of the way swallows take to human society, do you know where our barn-swallows came from? They were originally cliff-dwellers away out West. The early explorers found enormous collections of their nests plastered all over the perpendicular cliffs and along the bluffs. Just as soon, however, as the country settled up and men put up barns these little cliff-dwellers, deserting rocks and bluffs, began building their bottle-shaped nests under the eaves. The swallows live on insects--including squash-bugs, stink-bugs, shield-bugs, and jumping plant-lice; and that's supposed to be one of the reasons for the curious fact that they left their ancient family seats--they found so many more insects about the barns and the farmer's fields and the gardens and the orchards. TINY SOIL MILLS OF THE BABY SWALLOW Haven't you often watched them and listened to them, diving and chattering around the barn in their busy season; that is to say, in the spring and summer time? Then the air is full of insects and is fairly woven with their darting wings. Some keep busy picking up the insects that are always hovering about in a barnyard, while others dash away to some near-by marsh or to the meadow or to the creek. Over the grain-fields they go, over the meadows and back again straight to the nest where downy babies are cheeping for them. The parents feed them, stop and chatter a moment, and then off they go. Follow that one down to the marsh. See how she flies high, round and round in circles, and then swoops for an insect. She missed him! Then she wheels, darts up--darts down--to right--to left. There, she's got him! Then off like an arrow to the nest. The soft-bodied insects are chosen and chewed up for the babies, while the parents eat the tougher ones. And to help digestion they give the babies little bits of gravel, although they don't use it themselves. So, in grinding up this gravel the baby birds help make soil before they are old enough to do any nest-building. [Illustration: THE SAND MARTIN AND HIS HOME IN THE BANK] You've noticed, of course, that all the swallows about a barn don't build under the eaves. Some build under the rafters inside the barn. That isn't just a matter of taste; it's family tradition. The eave-builders are descendants of the cliff-swallows, while the birds known to bird students as "barn" swallows build under the rafters. But they don't take to the fine, new modern barns--all spick and span--the barn-swallows don't. If there's an old gray barn with doors that never shut quite snug, a board off here and there, and several panes in the cob-webbed windows broken out----