The Tree-Martin is more slender and has a greater extent of wing than the Purple Martins; and, instead of the beautiful dark purple (their prevailing colour), its entire upper plumage is dusky brown, the under surface white. But if these differences of structure and hue merely serve to show that it is not a very near relative of the other species, those exhibited in its habits remove it very far indeed from them. The Tree-Martin is a very garrulous bird, and no sooner arrives early in September, than we are apprised of the circumstance by the notes which the male and female incessantly sing in concert, fluttering and waving their wings the while, and seeming quite beside themselves with joy at their safe arrival; for invariably they arrive already mated. Their language is more varied, the intonation bolder and freer than that of our other Swallows. The length of the notes can be varied at pleasure; some are almost harsh, others silvery or liquid, as of trickling drops of water; they all have a glad sound; and many have that peculiar character of some bird-notes of shaping themselves into words. This Martin is never seen to alight on the ground or on the roofs of houses, but solely on trees; and when engaged in collecting materials for its nest, it sweeps down and snatches up a feather or straw without touching the surface. It breeds only in the clay-ovens of the Oven-bird (_Furnarius rufus_). I, at least, have never seen them breed in any other situation after observing them for a great many summers. An extraordinary habit! for, many as are the species that possess the parasitical tendency of breeding in other birds' nests, none of them confine themselves to the nest of a single species excepting the bird I am describing. It must, however, be understood that my knowledge of this bird has been acquired in Buenos Ayres, where I have observed it; and as this Martin possesses a wider range in South America than the Oven-birds, it is more than probable that in other districts it builds in different situations. On arriving in spring each pair takes up its position on some tree, and usually on a particular branch; a dead branch extending beyond the foliage is a favourite perch. Here they spend much of their time, never appearing to remain long absent from it, and often, when singing their notes together, fluttering about it with a tremulous uncertain flight, like that of a hovering butterfly. About three weeks after first arriving they begin to make advances towards the Oven-bird's nest that stands on the nearest post or tree; and if it be still occupied by the rightful owners, after much time has been spent in sporting about and reconnoitring it, a feud begins which is often exceedingly violent and protracted for many days.