Nesting.—Two nests, both in the Chisos Mountains, have been described. The first (discovered in 1932) was lodged between small rocks and deeply imbedded in dead oak leaves on the sloping bank of a dry stream bed. A dense ground cover of vines and other herbaceous plants arched completely over it, leaving an entrance only on the northwest side, toward the stream. The nest had a basic structure of loosely woven fine grasses, the outside reinforced with pieces of green moss and the rim with strips of cedar bark; the cavity (5 centimeters across the rim and 4 centimeters deep) was lined with fine grass, a little fur, and a few hairs (Van Tyne, 1936). The other nest, which was “on the ground, under a little bunch of oak leaves, at the edge of a talus slope, almost at the very base of the cliffs” (Sutton, 1935), was similar, but its basic structure included dry leaves, and the site was concealed by only a partial canopy of leaves (Van Tyne and Sutton, 1937).
Nest building was observed in the Chisos Mountains on May 7, 1932 (Van Tyne, 1936):
As I was crossing the dry stream bed about a hundred yards below Boot Spring, I suddenly saw within twenty-five feet of me a female warbler with nest material in her bill. I stopped instantly and, remaining motionless, was greatly relieved to see the warbler continue undisturbed by my presence. In a moment she dropped to the ground and entered the nest, which was on the sloping right bank of the stream about six feet back from the margin of the rocky stream bed. After working for about twenty seconds the warbler left the nest and flew down the stream bed a hundred and fifty feet. In twelve minutes she was back with more nest material to repeat the performance. Subsequent excursions for building material during the ensuing hour were of three, twelve, six, and twenty-two minutes’ duration. Each time she worked at the nest only fifteen to twenty seconds, until the last trip (at 11:43 A. M.) when she worked about two minutes and then departed, probably to feed, for she did not return again while I watched. Each trip to the nest had been made undeviatingly, without any hesitation, from the stream bed or from the forest to the west. Alighting almost directly above the nest, without a pause she dropped through the branches by three or four stages and promptly entered the nest, placed the material, and snuggled down working it into place. After a few moments she seemed to have completed this to her satisfaction, and, leaving the nest, she flew up to the branches ten or twelve feet above, fed for a few moments on the insects among the fresh green leaves of the little oaks and maples, and went away for more material.
When it was evident that the nest building was over for the time I went over to the nest and, examining it more closely, found that it was nearly built. The following day, May 8, it seemed to be finished.
Eggs.—Two complete clutches have been found, each containing four eggs. Four eggs collected and measured were 18 by 13.3, 18 by 13.5, 18 by 13.5, and 18.5 by 14 millimeters. They were creamy white, speckled, and blotched in a wreath at the larger end with “vinaceous fawn,” “light brownish drab,” and “cinnamon drab.”
Egg laying, in the one instance observed, was at daily intervals (May 12-15); the first egg was laid four days after completion of the nest. Incubation had begun May 16, the day after the last egg was laid. The length of the incubation period is not known. Females collected on May 12 (Peet), May 17 (Van Tyne), and May 20 (Sutton) had well-marked incubation patches; males collected at the same time had no patch.
Plumages.—The Colima warbler differs from its nearest relative, the Virginia’s warbler, in being larger; darker, less gray, above; crown paler; rump and upper tail coverts darker and richer in color; yellow of throat and breast absent or, if present, more green and more diffuse; sides and flanks more brownish; crissum darker, more aniline yellow; sexes much more nearly alike. The adult female Colima warbler is slightly darker than the male and is more brown below. It is apparently never yellow on the breast.
The juvenal plumage (known from only one specimen) differs from the adult plumage in lacking the crown spot and in having two buffy wing bars. The rump is also much more yellow (less green) and the crissum is more yellow (less orange). The young Colima warbler differs from the young Virginia’s warbler in having a larger bill, darker plumage, and a less ochraceous rump.
The fall plumage differs from that of the spring in being “darker and browner throughout, the gray of head a good deal obscured by deep olive or light brownish olive; crown patch orange rufous; under parts darker with whitish area in middle of belly more distinct and under tail-coverts duller, more nearly aniline yellow” (Bangs, 1925).