_Description._--Above glossy dark green; rump white; quills black, washed with green; upper tail-coverts dark green; tail-feathers black with greenish gloss; base of forehead white, extending a little backward over the lores; cheeks and whole under surface white; flanks and sides washed with smoky brown; axillaries and under tail-coverts pale smoky brown; bill and feet black: total length 5·5 inches, wing 4·45, tail 2·0. _Female_ similar. _Hab._ Brazil, Paraguay, and Argentina. This is the most abundant and best known of our Swallows; a pretty bird in its glossy coat of deep green, and rump and under surface snowy white; exceedingly restless in its disposition, quick and graceful in its motions; social, quarrelsome, garrulous, with a not unmusical song, beginning with long, soft, tremulous notes, followed by others shorter and more hurried, and sinking to a murmur. They are the last of all our migrants to leave us in autumn, and invariably reappear in small numbers about the houses on every warm day in winter. Probably many individuals in Buenos Ayres remain through the winter in sheltered situations, to scatter over the surrounding country whenever there comes a warm bright day. I once saw three together, skimming over the plains, on one of the coldest days I ever experienced on the pampas, the thermometer having stood at 29° Fahr. that morning. Further south their migration is more strict; and on the Rio Negro, in Patagonia, from March to August I did not meet with a single individual. In Buenos Ayres the autumnal migration of the Hirundines begins about the middle of February, and from that date vast numbers of this Swallow are seen travelling north, and, in some seasons, they continue passing for over a month. One autumn, in April, several days after the Swallows had all disappeared, flocks of the Common Swallow began again to appear flying north, and for ten days afterwards they continued to pass in large numbers. They would stoop to dip themselves in a pool where I observed them, and then alight on the reeds and bushes to rest, and appeared quite tired with their journey, rising reluctantly when approached, and some allowing me to stand almost within arm's length of them without stirring. I had never before observed any later or supplementary migration like this; for, as a rule, the causes which in some years delay the departure of birds seems to affect them all alike. Possibly these late birds come from some remote district, where exceptionally cold weather had retarded breeding-operations.