Though generally regular in their habits and instincts, yet the birds sometimes seem as whimsical and capricious as superior beings. One is not safe, for instance, in making any absolute assertion as to their place or mode of building. Ground-builders often get up into a bush, and tree-builders sometimes get upon the ground or into a tussock of grass. The song-sparrow, which is a ground builder, has been known to build in the knot-hole of a fence rail, and a chimney swallow once got tired of soot and smoke, and fastened its nest on a rafter in a hay barn. A friend tells me of a pair of barn swallows which, taking a fanciful turn, saddled their nest in the loop of a rope that was pendent from a peg in the peak, and liked it so well that they repeated the experiment next year. I have known the social-sparrow, or "hair-bird," to build under a shed, in a tuft of hay that hung down, through the loose flooring, from the mow above. It usually contents itself with half a dozen stalks of dry grass and a few long hairs from a cow's tail loosely arranged on the branch of an apple-tree. The rough-winged swallow builds in the wall and in old stone heaps, and I have seen the robin build in similar localities. Others have found its nest in old, abandoned wells. The house-wren will build in anything that has an accessible cavity, from an old boot to a bomb-shell. A pair of them once persisted in building their nest in the top of a certain pump-tree, getting in through the opening above the handle. The pump being in daily use, the nest was destroyed more than a score of times. This jealous little wretch has the wise forethought, when the box in which he builds contains two compartments, to fill up one of them, so as to avoid the risk of troublesome neighbors. The less skillful builders sometimes depart from their usual habit, and take up with the abandoned nest of some other species. The blue jay now and then lays in an old crow's-nest or cuckoo's-nest. The crow-blackbird, seized with a fit of indolence, drops its eggs in the cavity of a decayed branch. I heard of a cuckoo that dispossessed a robin of its nest; of another that set a blue jay adrift. Large, loose structures, like the nests of the osprey and certain of the herons, have been found with half a dozen nests of the blackbird set in the outer edges, like so many parasites, or, as Audubon says, like the retainers about the rude court of a feudal baron.