_Hylocichla_, BAIRD (s. g.), Rev. Am. Birds, 1864, 12. (Type, _Turdus
mustelinus_.)
[Line drawing: _Turdus mustelinus._
1570]
The essential characters of _Hylocichla_ have already been given. The
subgenus includes the small North American species, with _Turdus
mustelinus_, Gm., at the head as type, which are closely connected on
the one side with _Catharus_, by their lengthened tarsi, and with
_Turdus_ by the shape of the wing. The bills are shorter, more
depressed, and broader at base than in typical _Turdus_, so much so
that the species have frequently been described under _Muscicapa_.
It is not at all improbable that naturalists may ultimately conclude
to consider the group as of generic rank.
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.
The _T. swainsoni_ is found in abundance westward to the western limit
of the Rocky Mountain system; in the latter region specimens at all
seasons have the olive of a clearer, more greenish shade than in any
Eastern examples; this clearer tint is analogous with that of the
Rocky Mountain form of _pallasi_ (_auduboni_). In precisely the same
region inhabited by the _pallasi_ var. _nanus_ the _swainsoni_ also
has a representative form,—the var. _ustulatus_. This resembles in
pattern the var. _swainsoni_, but the olive above is decidedly more
rufescent,—much as in Rocky Mountain specimens of _T. fuscescens_;
the spots on jugulum and breast are also narrower, as well as hardly
darker in color than the back; and the tail is longer than in Rocky
Mountain _swainsoni_, in which latter it is longer than in Eastern
examples. The remaining species—_mustelinus_, _fuscescens_, and
_aliciæ_—extend no farther west than the Rocky Mountains; the first
and last only toward their eastern base, while the second breeds
abundantly as far as the eastern limit of the Great Basin.
The _T. fuscescens_, from the Rocky Mountains, is considerably darker
in color above, while the specks on the throat and jugular are sparser
or more obsolete than in Eastern birds.
In _T. mustelinus_, the only two Western specimens in the collection
(Mount Carroll, Ills., and Fort Pierre) have the rump of a clearer
grayish than specimens from the Atlantic Coast; in all other respects,
however, they appear to be identical. Some Mexican specimens, being in
winter plumage, have the breast more buffy than Northern (spring or
summer) examples, and the rufous of the head, etc. is somewhat
brighter.
In _aliciæ_, no difference is observed between Eastern and Western
birds; the reason is, probably, that the breeding-ground of all is in
one province, though their migrations may extend over two. There is,
however, a marked difference between the spring and autumn plumage;
the clear grayish of the former being replaced, in the latter, by a
snuffy brown, or sepia tint,—this especially noticeable on wings and
tail.
[Illustration: PLATE I.
1. Turdus mustelinus, _Gm._ Penn., 1570.
2. “ ustulatus, _Nutt._ Oregon, 2040.
3. “ aliciæ, _Baird_. Illinois, 10084.
4. “ swainsoni, _Cab._ Penn., 981.
5. “ fuscescens, _Steph._ D. C., 28231.
6. “ pallasii, _Cab._ Penn., 2146.
7. “ nanus, _Aud._ Cala., 17997.
8. “ auduboni, _Baird_. Rocky Mts., 10886.]
The following synopsis is intended to show the characters of the
different species and varieties.