Plumages.—The molts and plumages are evidently similar to those of the orange-crowned warbler, though the lutescent is, of course, decidedly more yellow in all plumages.
Food.—Prof. Beal (1907) examined the contents of the stomachs of 65 California specimens of this species.
Less than 9 percent of the food is vegetable matter, and is made up of 3 percent of fruit and rather more than 5 percent of various substances, such as leaf galls, seeds, and rubbish. Fruit was found in only a few stomachs, but the percentage in each was considerable; figs were the only variety identified. [Of the 91 percent animal matter,] Hemiptera are the largest item and amount to over 25 percent, mostly leaf-bugs, leaf-hoppers, plant-lice, and scales. Plant-lice were found in only one stomach and scales in 5, of which 3 contained the black olive species. Beetles amount to about 19 percent of the food, and with the exception of a few Coccinellidae are of harmful families, among which are a number of weevils. * * * Caterpillars are eaten rather irregularly, though they aggregate 24 percent for the year. Stomachs collected in several months contained none, while in others they amounted to more than half of the food. * * * Hymenoptera amount nearly to 15 percent, and are mostly small wasps, though some ants are eaten.
Other items were flies, less than 1 percent, and spiders, 7 percent. W. L. McAtee (1912) says that this is one of only two wood warblers known to prey upon codling moths. “The lutescent warbler shows a strong liking for the pupae, two taken in California in May having eaten 10 and 18 pupae, respectively.”
Behavior.—Mrs. Wheelock (1904) writes thus of its feeding activities: “All day long he flits about through the oak trees, leaning away over the tips of the boughs to investigate a spray of leaves, or stretching up his pretty head to reach a blossom just above him; now clinging head downward underneath a spray, or hovering under the yellow tassels as a bee hovers beneath a flower.”
Voice.—Samuel F. Rathbun (MS.) gives me his impression of the song of the lutescent warbler as follows: “Its song is a succession of trilling notes on a slightly rising then falling key, the latter more lightly given and faster. There is an apparent ease in this song that is suggestive of airiness, and, although simple in construction, it is pleasing to hear and further bears the stamp of distinctiveness.”
Fall.—The fall migration is southward to southern California, western Mexico, and Guatemala. The movement is apparently leisurely and quite prolonged, for the earliest birds begin leaving western Washington in August and September, and Theed Pearse gives me two October dates for Vancouver Island, with his latest date November 1. Taylor and Shaw (1927) write of the fall movement on Mount Rainier as follows:
The postnuptial scatter movement was in full swing by the middle of August. At this time the lutescent warbler was often found in the same flocks with Shufeldt juncos, western golden-crowned kinglets, or chestnut-backed chickadees. It is not unlikely that there is some good reason for this flocking, aside from the companionship involved. The warblers and the juncos, kinglets, or chickadees probably do not compete for food as would one warbler with another of the same species. The individual warbler, attached to a flock of kinglets, let us say, may be the more surely guided to available food. Then, too, differences in alertness of the two or more species concerned may afford greater protection to each than would be the case if they remained separate.
Robert Ridgway (1877) met with these warblers in large numbers in Nevada:
In the fall, the thickets and lower shrubbery along the streams, particularly those of the lower cañons, would fairly swarm with them during the early portion of the mornings, as they busily sought their food, in company with various insectivorous birds, especially the Black-capped Green Warbler (Myiodioctes pusillus) and Swainson’s Vireo (Vireosylvia swainsoni). At such times they uttered frequently their sharp note of chip. The brightly-colored specimens representing H. lutescens were prevalent in the western depression of the Basin, but were not observed eastward of the upper portion of the Valley of the Humboldt, nor at any locality during the summer; and wherever found, were associated with individuals of the other form, which is the only one found breeding on the mountains. It is therefore inferred that all these individuals were migrants from the northern Pacific Coast region and the Sierra Nevada, while those of H. celata proper were from the higher portions of the more eastern mountains, or from farther northward in the Rocky Mountain ranges, full-fledged young birds being numerous in the high aspen woods of the Wasatch Mountains in July and August.