The nests measure about 4 inches in diameter and 2¾ in height. The cavity is 2 inches deep, and its diameter 2½ inches. They are unusually compact for the nest of a thrush, and are composed chiefly of an elaborate interweaving of fine sedges, leaves, stems of the more delicate Equisetaceæ, dry grasses, strips of fine bark, and decayed leaves, the whole intermingled with the paniculated inflorescence of grasses. There is little or no lining other than these materials. These nests were all found, with but few exceptions, on the branches of low trees, from two to seven feet from the ground. In a few exceptional cases the nests were built on the ground.
Occasionally nests of this species are found constructed with the base and sides of solid mud, as with the common Robin (Turdus migratorius). In these, as also in some other cases, their nests are usually found on or near the ground. So far as I am aware neither its occasional position on the ground, nor its mud frames, are peculiarities ever noticeable in nests of T. swainsoni.
The eggs were usually four in number. Their color is either a deep green tint, or green slightly tinged with blue; and they are marked with spots of russet and yellowish-brown, varying both in size and frequency. Their mean length is .92 of an inch, and their mean breadth .64. The maximum length is .94 and the minimum .88 of an inch. There is apparently a constant variation from the eggs of the T. swainsoni; those of the aliciæ having a more distinctly blue ground color. The nests are also quite different in their appearance and style of structure. The Hypnum mosses, so marked a feature in the nests of T. swainsoni, as also in those of T. ustulatus, are wholly wanting in those of T. aliciæ.
This bird and the robin are the only species of our thrushes that cross the Arctic Circle to any distance, or reach the shore of the Arctic Ocean. It occurs from Labrador, all round the American coast, to the Aleutian Islands, everywhere bearing its specific character as indicated above. It is extremely abundant on and near the Arctic coast, between the mouth of
the Mackenzie River and the Coppermine, more than 200 specimens (mostly with their eggs) having been sent thence to the Smithsonian Institution by Mr. MacFarlane. In all this number there was not a single bird that had any approach to the characters of T. swainsoni, as just given. From the Slave Lake region, on the other hand, T. swainsoni was received in nearly the same abundance, and unmixed during the breeding season with T. aliciæ.
Turdus swainsoni, Cabanis.
OLIVE-BACKED THRUSH; SWAINSON’S THRUSH.
Turdus swainsoni, Cab. Tschudi, Fauna Peruana, 1844-46, 188.—? Sclater & Salvin, Ibis, 1859, 6 (Guatemala).—Sclater, P. Z. S. 1858, 451 (Ecuador); 1859, 326.—Ib. Catal. 1861, 2, No. 11.—Baird, Birds N. Am. 1858, 216; Rev. Am. B., 1864, 19.—Gundlach, Cab. Jour. 1861, 324 (Cuba).—Ib. Repert. 1865, 229.—Pelzeln, Orn. Brazil. II. 1868, 92 (Marambitanas, Feb. and March).—Lawr. N. Y. Lyc. IX, 91 (Costa Rica).—Ridgway.—Maynard.—Samuels, 152.—Cooper, Birds Cal. 6.—Dall & Bannister. Turdus minor, Gmelin, Syst. Nat. I, 1788, 809 (in part). Turdus olivaceus, Giraud, Birds L. Island, 1843-44, 92 (not of Linn.). (?) Turdus minimus, Lafresnaye, Rev. Zoöl. 1848, 5.—Sclater, P. Z. S. 1854, 111.—Bryant, Pr. Bost. Soc. VII, 1860, 226 (Bogota).—Lawrence, Ann. N. Y. Lyc. 1863. (Birds Panama, IV, No. 384.)
Sp. Char. Upper parts uniform olivaceous, with a decided shade of green. The fore part of breast, the throat and chin, pale brownish-yellow; rest of lower parts white; the sides washed with brownish-olive. Sides of the throat and fore part of the breast with sub-rounded spots of well-defined brown, darker than the back; the rest of the breast (except medially) with rather less distinct spots that are more olivaceous. Tibiæ yellowish-brown. Broad ring round the eye, loral region, and a general tinge on the side of the head, clear reddish buff. Length, 7.00; wing, 4.15; tail, 3.10; tarsus, 1.10.
Hab. Eastern North America; westward to Humboldt Mountain and Upper Columbia; perhaps occasionally straggling as far as California; north to Slave Lake and Fort Yukon; south to Ecuador and Brazil. Cuba, Gundlach; Costa Rica, Lawr.