The rarest of the species are Swainson's warbler, said to be disappearing; the cerulean warbler, said to be abundant about Niagara; and the mourning ground warbler, which I have found breeding about the head-waters of the Delaware, in New York. The vireos, or greenlets, are a sort of connecting link between the warblers and the true fly-catchers, and partake of the characteristics of both. The red-eyed vireo, whose sweet soliloquy is one of the most constant and cheerful sounds in our woods and groves, is, perhaps, the most noticeable and abundant species. The vireos are a little larger than the warblers, and are far less brilliant and variegated in color. There are four species found in most of our woods, namely, the red-eyed vireo, the white-eyed vireo, the warbling vireo, and the solitary vireo,--the red-eyed and warbling being most abundant, and the white-eyed being the most lively and animated songster. I meet the latter bird only in the thick, bushy growths of low, swampy localities, where, eluding the observer, it pours forth its song with a sharpness and a rapidity of articulation that are truly astonishing. This strain is very marked, and, though inlaid with the notes of several other birds, is entirely unique. The iris of this bird is white, as that of the red-eyed is red, though in neither case can this mark be distinguished at more than two or three yards. In most cases the iris of birds is a dark hazel, which passes for black. The basket-like nest, pendent to the low branches in the woods, which the falling leaves of autumn reveal to all passers, is, in most cases, the nest of the red-eyed, though the solitary constructs a similar tenement, but in much more remote and secluded localities. The general color of this group of birds is very light ash beneath, becoming darker above, with a tinge of green. The red-eyed has a crown of a bluish tinge. Most birds exhibit great alarm and distress, usually with a strong dash of anger, when you approach their nests; but the demeanor of the red-eyed, on such an occasion, is an exception to this rule. The parent birds move about softly amid the branches above, eying the intruder with a curious, innocent look, uttering, now and then, a subdued note or plaint, solicitous and watchful, but making no demonstration of anger or distress.