This diminutive dark-plumaged species is the smallest of our Hirundines. In Buenos Ayres they appear early in September, arriving before the Martins, but preceded by the Common Swallow. They are bank-birds, breeding in forsaken holes and burrows, for they never bore into the earth themselves, and are consequently not much seen about the habitations of man. They sometimes find their breeding-holes in the banks of streams, or, in cultivated districts, in the sides of ditches, and even down in wells. But if in such sites alone fit receptacles for their eggs were met with, the species, instead of one of the commonest, would be rare indeed with us; for on the level pampas most of the water-courses have marshy borders, or at most but low and gently sloping banks. But the burrowing habits of two other animals--the Vizcacha (_Lagostomus trichodactylus_), the common large rodent of the pampas, and the curious little bird called Minera (_Geositta cunicularia_)--have everywhere afforded the Swallows abundance of breeding-places on the plains, even where there are no streams or other irregularities in the smooth surface of the earth. The Minera bores its hole in the sides of the Vizcacha's great burrow, and in this burrow within a burrow the Swallow lays its eggs and rears its young, and is the guest of the Vizcacha, and as much dependent on it as the House-Wren and the Domestic Swallow on man; so that in spring, when this species returns to the plains, it is in the villages of the Vizcachas that we see them. There they live and spend the day, sporting about the burrows, just as the Common Swallow does about our houses; and to a stranger on the pampas one of these villages, with its incongruous bird and mammalian inhabitants, must seem a very curious sight in the evening. Before sunset the old male Vizcachas come forth to sit gravely at the mouths of their great burrows. One or two couples of Mineras, their little brown bird-tenants, are always seen running about on the bare ground round the holes, resting at intervals with their tails slowly moving up and down, and occasionally trilling-out their shrill laughter-like cry. Often a pair of Burrowing-Owls also live in the village, occupying one of the lesser disused burrows; and round them all flit half a dozen little Swallows, like twilight-moths with long black wings. It is never quite a happy family, however, for the Owls always hiss and snap at a Vizcacha if he comes too near; while the little Swallows never become reconciled to the Owls, but perpetually flutter about them, protesting against their presence with long complaining notes.