During the summer season the Audubon Warbler keeps mainly to coniferous trees, foraging from 10 to 50 feet or more above the ground. In the Transition Zone and part of the Canadian Zone it shares this habitat with the Hermit Warbler, but at the higher altitudes it is the only warbler present in the evergreen forest.
Farther north, in Mono County, Calif., James B. Dixon tells me that he found it nesting between 7,600 and 9,500 feet elevation. Referring to the Toyabe region in Nevada, Dr. Jean M. Linsdale (1938) found the Rocky Mountain form in a somewhat different environment: “In the mountains the area occupied by this warbler agreed fairly well with the area covered with trees. Individuals were seen most often in aspens, limber pines, birches, willows, and mountain mahoganies.” Angus M. Woodbury (MS.) says of the breeding range of the Rocky Mountain form in Utah: “It summers in altitudes ranging from about 7,000 to 10,000 feet and nests in almost any of the components of the forests in those altitudes; pine, fir, spruce, aspen, or oak.”
In Washington, Audubon’s warbler is common and well distributed from near sea level in the vicinity of Seattle and Tacoma up to about 8,000 feet in the mountains. Near Tacoma, D. E. Brown showed us some typical lowland haunts of this warbler in the so-called “prairie region.” On this smooth, flat land, a fine growth of firs and cedars was scattered about in the open; the two or three local species of firs were most abundant and were growing to perfection, being well branched down to the ground.
Spring.—There is a northward as well as an altitudinal migration in the spring. Samuel F. Rathbun says in his notes from Seattle: “Although the Audubon’s is of frequent if not regular occurrence during the winter, a migration of the bird through the region is to be noted each spring and fall.” Near Seattle the first birds are seen and their song is heard about March 10 to 15, and numbers are seen passing through up to the latter part of April. “By way of comparison, in the Lake Crescent section the first are seen about April 2, at the earliest, and after three weeks the last appear to have passed by, as the species performs its spring migration in a leisurely manner.” A later wave of migrants passes through Seattle between April 10 and 25, probably birds that nest farther north.
Migration is evident in Utah, for Woodbury (MS.) says: “In addition to its summer residence, it is a common migrant through the state, and a sparse winter resident, mainly at low altitudes. It migrates through the streamside and cultivated trees of the valleys, including shade trees and orchards. The migrations cover a period of about 6 weeks each in spring and fall, usually from about mid-April to the end of May and from mid-September to the end of October, but in different years the waves may be a little earlier or later.”
In California, there is a gradual exodus of Audubon’s warblers from the lowlands to the mountains during April and May. Mrs. Amelia S. Allen tells me that “by the end of April they have disappeared from the San Francisco Bay region.” And Swarth (1926b) says that in May, following the spring molt, “there is a gradual withdrawal of the birds to the higher mountains and to more northern latitudes.”
Audubon’s warbler occurs abundantly on the Huachuca Mountains, Ariz., but as a migrant only, during March, April, and May. Swarth (1904) writes:
Though distributed over all parts of the mountains, they were at all times more abundant in the higher pine region, than elsewhere; and on April 24, 1903, I found them particularly numerous along the divide of the mountains, evidently migrating. They could hardly be said to be in flocks on this occasion, for along the ridge, which runs almost due north and south, there was for several miles a continuous stream of Audubon Warblers travelling rapidly from tree to tree, always moving in a northerly direction; sometimes a dozen or more in one pine, and sometimes only two or three, but never stopping long and all moving in the same direction. Almost all that were seen on this occasion were high plumaged males, hardly half a dozen females being observed for the day.