From its larger size, brighter colour, and finer song, the Wood Warbler deserves to be first noticed; and the first step should be to distinguish it from its congeners. Perhaps none of the small insectivorous birds have been more confounded one with another than have the members of this group, not only by observers of the living birds, but by naturalists with skins of each before them. Taking the three species which annually visit us—_i. e._, the Wood Warbler, the Willow Warbler, and the Chiff-chaff—it will be found on comparison that they differ in size as follows— Length. Wing. Tarsus. Wood Warbler 5·2 in. 3·0 in. 0·7 in. Willow Warbler 5·0 ” 2·6 ” 0·7 ” Chiff-chaff 4·7 ” 2·4 ” 0·6 ” Not only is the Wood Warbler the largest of the three, but it has comparatively the longest wings and the longest legs. The wings, when closed, cover three-fourths of the tail. In the Willow Wren, under the same circumstances, less than half the tail is hidden. The Chiff-chaff’s wing is shorter again. In my edition of White’s “Selborne,” founded upon that of Bennett, 1875, pp. 56, 57, will be found a long footnote on the subject, with woodcuts illustrating the comparative form of the wing in these three birds. Mr. Blake-Knox, in “The Zoologist” for 1866, p. 300, has pointed to the second quill-feather, depicted in a sketch accompanying his communication, as being an unfailing mark of distinction.[6] When we reflect, however, upon the variation which is found to exist in the length of feathers, owing to the age of the bird, moult, or accident, too much stress ought not to be laid upon this as a character. At the same time there is no doubt that, taken in connection with other details, it will often assist the determination of a species. After examining a large series of these birds, I have come to the conclusion that, as regards the wings, the following formulæ may be relied on: Wood Warbler, 2nd=4th; 3rd and 4th with outer webs sloped off towards the extremity. Willow Warbler, 2nd=6th; 3rd, 4th, and 5th sloped off. Chiff-chaff, 2nd=7th; 3rd, 4th, 5th, and 6th sloped off. The Wood Warbler is much greener on the back and whiter on the under parts than either of its congeners, and has a well-defined superciliary streak of sulphur-yellow, which, in the Willow Wren, is much shorter and paler. The legs of the Wood Warbler and Willow Wren are brownish flesh-colour, while those of the Chiff-chaff are dark brown. After the first moult, the young of all three species are much yellower in colour than their parents. Hence the mistake which Vieillot made in describing the young of _P. trochilus_ as a distinct species under the name of _flaviventris_.