I found it fairly common in the upper reaches of Ramsey Canyon in the Huachuca Mountains of Arizona, among the tall, scattered yellow pines, at elevations between 6,000 and 7,000 feet, where a nest with young was found on June 4, 1922. Swarth (1904) found it more common there as a migrant than as a breeding bird and rather irregular in its abundance.
Grace’s warbler, now well-known as a summer resident in the mountains of southern Colorado, New Mexico, Arizona, Sonora, and Chihuahua, is apparently closely related to the yellow-throated warbler of the southern States and to Adelaide’s warbler of Puerto Rico; it has a slightly differentiated subspecies in Central America.
Nesting.—What is probably the first authentic nest of Grace’s warbler to be reported was taken in Yavapai County, Ariz., on June 23, 1890, by H. Keays for H. P. Attwater. This nest is described by Samuel B. Ladd (1891) as “placed on limb of pine sixty feet from the ground. Nest very compact; outside diameter 3 in. by 11⁄2 in. high; inside diameter 13⁄4 in. by 11⁄4 in. deep. The body of this nest is composed of horse-hair, strings and vegetable fibres. The most abundant vegetable material interwoven consists of the staminate catkins and bud scales of Quercus emoryi. There is also some wool, vegetable down, and insect webbing, in which are entangled the exuviae of some caterpillar. Attached on the outside was a small staminate cone of a species of Pinus. Nest well lined with feathers and horse-hair.”
O. W. Howard (1899) found two nests in Arizona; one nest was “placed deep down in the middle in a large bunch of pine needles and was entirely hidden from view.” The other he found “in a red fir tree. It was placed in a thick bunch of leaves at the extremity of a limb about fifty feet from the ground.” A nest with four eggs, in the Doe Museum in Gainesville, Fla., taken by O. C. Poling on May 25, 1891, at 8,000 feet in the Huachuca Mountains, was built in a bunch of pine needles and cones at the end of a long branch of a red pine, 20 feet from the ground.
Eggs.—From 3 to 4 eggs, apparently more often 3, make up the set for Grace’s warbler. They are ovate with a tendency toward elongate ovate, and are only slightly glossy. They are white or creamy white, finely speckled and spotted with “auburn,” “bay,” or “chestnut brown,” intermingled with “light brownish drab,” “deep brownish drab,” or “pale vinaceous drab.” The markings are concentrated at the large end, where they frequently form a distinct wreath, leaving the lower half of the egg immaculate. Occasionally eggs are speckled all over; and some are marked with blotches. Generally the drab spots are in the majority, when the fewer brown spots, which are often as dark as to appear almost black, are more prominent. The measurements of 38 eggs average 16.9 by 12.7 millimeters; the eggs showing the four extremes measure 18.2 by 13.1, 18.0 by 13.3, 14.8 by 12.7, and 15.4 by 11.7 millimeters (Harris).
Young.—Nothing seems to have been published on incubation or on the development and care of the young.
Plumages.—Ridgway (1902) describes the young male in first plumage as “above plain grayish hair brown or drab-gray, the feathers ash gray beneath the surface; sides of head similar but rather paler; malar region, chin, and throat pale brownish gray, minutely and sparsely flecked with darker, the chest similar, but with rather large roundish spots of dusky; rest of under parts dull white streaked or spotted with dusky gray medially, dull grayish laterally.”
Swarth (1904) writes of the postjuvenal molt:
A young male taken July 13th is in the brown streaked plumage, but yellow feathers are beginning to appear along the median line of the throat and upper breast, and the yellow superciliary stripe is also beginning to show. Another, a little older, has the streaks of the lower parts restricted to the sides and flanks, and the yellow markings nearly perfect. A male taken on July 30th, which has just discarded the juvenile for the winter plumage, differs from the autumnal adults in having the white of the under parts more strongly tinged with buff; and whereas the adult has the back decidedly streaked, though the markings are overcast by the brownish edgings to the feathers, in the juvenile these markings are but imperfectly indicated.
Apparently, the nuptial plumage is assumed by wear alone, for no available specimens show any signs of prenuptial molt and both young birds and adults in the fall are much like the spring birds, but browner and with the markings obscured by brownish tips that probably wear away before spring.