In this group there appears to be five well-marked forms or “species.” They are, _mustelinus_, Gm., _pallasi_, Caban., _fuscescens_, Steph., _swainsoni_, Caban., and _aliciæ_, Baird. The first-named is totally unlike the rest, which are more closely related in appearance. In studying carefully a very large series of specimens of all the species, the following facts become evident:— 1. In autumn and winter the “olive” color of the plumage assumes a browner cast than at other seasons; this variation, however, is the same in all the species (and varieties), so that in autumn and winter the several species differ from each other as much as they do in spring and summer. [Illustration: _Turdus ustulatus._] Of these five species, two only (_pallasi_ and _swainsoni_) inhabit the whole breadth of the continent; and they, in the three Faunal Provinces over which they extend, are modified into “races” or “varieties” characteristic of each region. The first of these species, as the _pallasi_ var. _pallasi_, extends westward to the Rocky Mountains, and migrates in winter into the South; specimens are very much browner in the winter than in spring; but in the Rocky Mountain region is a larger, grayer race, the var. _auduboni_. This, in its migrations, extends along the central mountain region through Mexico to Guatemala; specimens from the northern and southern extremes of this range are identical in all the specific characters; but the southern specimens, being in the fall and winter dress, are browner in color than northern ones (spring birds); an autumnal example from Cantonment Burgwyn, N. M., is as brown as any Central American specimen. Along the Pacific Province, from Kodiak to Western Mexico, and occasionally straggling eastward toward the Rocky Mountain system, there is the var. _nanus_, a race _smaller_ than the var. _pallasi_, and with much the same colors as var. _auduboni_, though the rufous of the tail is deeper than in either of the other forms. In this race, as in the others, there is no difference in size between specimens from north and south extremes of its distribution, because the breeding-place is in the North, all Southern specimens being winter sojourners from their Northern birthplace.