Nesting.—Of its nesting habits, J. Stuart Rowley continues: “I have found dusky warblers nesting in every conceivable sort of place, ranging from those placed on the ground in the grass to those placed 15 feet up in toyon trees. The usual nesting site here seems to be in a small toyon bush, rather well concealed, but not over 2 to 3 feet from the ground; the nests are made of fibres and grasses and, although nicely cupped and lined, are rather bulky affairs externally for a warbler to build.” Howell (1917) writes:
The usual nesting site of the Lutescent Warbler is on the ground, but I have never heard of sordida building in such a situation. On the smaller barren islands, such as the Coronados and Todos Santos (where it is common), they build in a bush or tangle of vines, a foot or so above the ground, and the nest is always mainly constructed of gray moss, where this is to be had, lined with a little fine grass. On the larger islands, where there are good-sized trees, the site chosen may be a thicket of vines several feet above the bed of a stream, a small shrub, say four feet up, or perhaps an oak as much as fifteen feet above the ground. In such case the nest is quite substantially made of leaves, twigs, bark, rootlets, and often a little sheep wool. Three or four eggs constitute a set, and at least two broods of young are raised each year.
A most unusual nesting site for a dusky warbler is described by Clinton G. Abbott (1926). It was—
a decorative fern basket inside a small lath house adjoining the home of Mrs. A. P. Johnson, Jr., at 2470 C Street, San Diego. * * * Her house is in one of the older residential sections of the city, known as Golden Hill. The homes here are large and surrounded by more or less extensive grounds, but the whole aspect is distinctly urban, with streets everywhere paved. Broadway, with double trolley tracks, is only one block away. The lath house, sixteen by twenty-four feet in size, was filled with a luxuriant growth of cultivated plants. A rectangular path within was marked at its corners by four wire fern baskets suspended about four feet from the ground. In one of these were the remains of the two previous years’ nests, and in the basket diagonally opposite was the inhabited nest, which contained three eggs. Although the eggs were manifestly not fresh, there was no bird about and they seemed cool to my touch. I waited about for fully ten minutes and was beginning to fear that disaster had overtaken the home, when I heard a low, scolding note overhead. Then down from between the slats hopped the dainty little warbler, and, with no concern whatsoever, she took her place upon the eggs, although I was standing in full view close by. [The nest was] cosily placed in the moss at the base of the ferns.
We soon discovered that not only was the bird practically fearless in the ordinary sense, but that she would even allow us to touch her without leaving her nest. She would permit us to raise her from her eggs with no greater protest than a pecking at the intruding finger. If she was not sitting sufficiently broadside for a good photograph, it was possible to arrange her the way we wanted her! Sometimes, if our familiarity was beyond her patience, she would merely hop among the foliage behind the nest, wait there for a few minutes, and then nestle back on her eggs.
Eggs.—Three eggs seem to constitute the average set for the dusky warbler, with occasionally only two or as many as four. Mr. Rowley tells me that, out of at least two dozen nests examined, he found only two sets of four; one nest had only one newly hatched young, and two or three nests held two well-incubated eggs. The eggs are apparently indistinguishable from those of the mainland races. The measurements of 27 eggs average 17.0 by 13.2 millimeters; the eggs showing the four extremes measure 18.5 by 13.5, 17.6 by 14.0, and 16.0 by 12.7 millimeters.
Winter.—Many of the dusky warblers, perhaps most of them, desert the islands in the fall when they become dry and uninviting, for the winter spreading widely on the mainland as far north as the San Francisco Bay region and inland to Merced County. Dr. Joseph Grinnell (1898) says: “This subspecies appears in the vicinity of Pasadena in the oak regions and along the arroyos in large numbers during August, and even by the middle of July. Remains in diminishing numbers through the winter; the latest specimen noted in the spring was secured by me, Feb. 29 (‘96).”