The nests were small and compact and well hidden in their cavity. Only twice did protruding material betray the location. In one case nesting material protruded from a woodpecker hole, and the other was a bulky nest that showed from each side of a split branch. This last nest I thought must belong to a House Finch, but investigation showed warbler ownership. Nests were made of bark, weeds, and mesquite leaf-stems, and lined with fine bark, horse and cow hair, a few feathers and sometimes a little rabbit fur. The site averaged six and one-half feet from the ground, the lowest being 18 inches and the highest 15 feet. * * *
In nest-building the female seems to do all the work, her mate sometimes accompanying her on trips to and from the tree, but more frequently flitting about the tops of adjacent trees, occasionally uttering his little warble. One pair I watcht had a nest in a Texas Woodpecker hole in a palo verde tree about 15 feet from the ground. The female brought material to the nest three times in two minutes, then a seven minute interval, followed by two trips in three minutes. The male accompanied her on two trips then made himself scarce. He indulged in no singing and both birds were silent, tho in many cases one or both gave the call note at intervals.
Others have mentioned nests of Lucy’s warblers in verdins’ nests, probably all old winter nests of the male verdin, relined to suit the warbler. O. W. Howard (1899) records such a nest and adds: “Other nests were placed in crevices along river banks where roots of trees were sticking out and one or two were found in natural cavities of the Giant Cactus, or in woodpecker holes therein.” We found a nest with young in a cavity in the bleached skeleton of a fallen giant cactus, where I set up my camera and took several photographs of the bird feeding the young. A very pretty nest of this warbler is in the Thayer collection in Cambridge; it was evidently built in the fork of a mesquite limb, supported by a cluster of old and fresh, green twigs; it is made externally of the leaves, petioles, fine green twigs, and flower clusters of the mesquite and is decorated with a few feathers of the white-winged dove; it is lined internally with fine fibers, white cows' hair and black horsehair, and more dove feathers; it measures 4 by 3 inches in outside diameter and 2 by 11⁄3 inside; the outside height is nearly 3 inches, and the inside cup is about 13⁄4 inches deep. A set in my collection was taken from a hole 3 feet above the base of a sandy bank along a wash near the San Pedro River, in Arizona.
Eggs.—Lucy’s warbler lays from 3 to 7 eggs, but the set usually consists of 4 or 5; the larger sets are rare, but O. W. Howard has found two sets of 7, and several sets of 6 have been recorded. The eggs are ovate to short ovate and have very little lustre. The white or creamy white ground color is finely speckled with shades of “chestnut,” “bay,” or “auburn.” The eggs that have markings in the darker shades of “chestnut” and “bay” frequently have a scattering of minute spots of “brownish drab” that are often lacking on eggs with the lighter markings of “auburn.” The spots are usually concentrated at the large end. The measurements of 50 eggs average 14.6 by 11.4 millimeters; the eggs showing the four extremes measure 16.5 by 11.5, 14.6 by 12.0, 13.2 by 11.2, and 13.7 by 10.7 millimeters. (Harris.)
Young.—The period of incubation seems to be unknown, and I can find no information on the development and care of the young. Evidence points to the conclusion that incubation and brooding are performed entirely by the female, and that at least two broods are reared in a season. Mr. Swarth (1905) says that “several broods are probably raised, as unfinished nests and incomplete sets were found at the same time that broods of young as large as the adults were seen flying about.”
Plumages.—Ridgway (1902) says that the young in juvenal plumage are “essentially like adults, but much clearer white beneath; no trace of chestnut on crown; upper tail-coverts ochraceous-buff instead of chestnut; middle and greater wing-coverts tipped with whitish or pale buffy, producing two rather distinct bars.” He might have added that the tertials are edged with cinnamon, and that the primaries and rectrices are edged and tipped with white.
There is apparently a partial postjuvenal molt, some time during the summer, when all the plumage except the flight feathers, remiges, and rectrices, is renewed. Young birds now become very similar to adults, but can be recognized by the juvenal wings and tail until the edgings wear off. I can find no evidence of a prenuptial molt in either young or old birds. I have seen adults in complete postnuptial molt in August. Fall birds are tinged with brown above and with pale brownish buff below; the chestnut crown patch is concealed by very broad brownish gray tips. Females are not always distinguishable from males, but usually the chestnut on the crown and upper tail coverts is paler and more restricted.
Food.—Nothing definite seems to have been published on the food of Lucy’s warbler, but it is evidently largely, if not wholly, insectivorous, as it is often seen foraging in the foliage and flower clusters of the mesquites and in other trees. Dr. W. P. Taylor tells me that he has seen it feeding on the pendant sprays of ocotillo flowers, probably gleaning insects or other materials from the exterior. In late spring when the mesquites, palo verdes, the various cacti, and even the saguaros burst into full bloom, these gorgeous desert plants are a blaze of color and attract myriads of insects.
Behavior.—Mr. Gilman (1909) says that “shyness about the nest seems to be characteristic of these birds.” He was seldom able to flush one from its nest. "In three cases only, did the parent birds show what might be called proper amount of solicitude when the nest was approacht. Some of them seemed rather touchy about their nests, leaving them if the nest were toucht even so lightly.” Some nests, with incomplete sets, were deserted after they had been inspected; but others were not. “They took good care not to sing in the nest tree, preferring to confine their performances to trees some distance away. The male would frequently meet me several rods from the nest and flit from tree to tree singing at short intervals. Once I made a complete circuit of the nest tree and he accompanied me the entire distance. This was an exceptional case of course. While going from tree to tree and singing, the bird usually tried to keep hidden as much as possible and was rather successful in the effort.”