Field marks.—The male in his gay spring plumage is not likely to be confused with any other warbler except the myrtle warbler, from which it differs in having a brilliant yellow throat instead of a white one; in other words, auduboni has five patches of yellow against four for coronata. In immature and fall plumages the two species are much alike, but auduboni has four or five large white patches on each side of the tail, while coronata has only two or three, in the different plumages; these white markings are diagnostic in any plumage. The yellow rump is always conspicuous at any season, even when the other yellow markings are more obscured.
Fall.—The fall migration is a reversal of the spring migration, from the north southward and from the mountains down to the valleys and lowlands. Rathbun tells me that the southward migrants pass through Washington during October and November, but that a few remain there and even farther north, in winter. In California, Audubon’s warblers that have bred in the mountains begin to drift downward to lower levels in August, the young birds coming first, so that by September they are well spread out over the lowlands almost down to sea-level. Soon after the first of October, the first of the migrants cross the border into Mexico on their way to winter quarters. Dr. Taylor tells me that in New Mexico during October these warblers are abundant in the aspens, being “by far the most numerous species of bird.”
Winter.—Audubon’s warbler is a hardy bird. At least some individuals remain in winter almost up to the northern limit of its breeding range; and while it retires entirely from its summer haunts in the mountains, most of its breeding range elsewhere is not wholly deserted. It probably remains as far north as it can make a living; its adaptability in finding a food supply helps in this and makes it one of the most successful of western birds as well as one of the most abundant in all parts of its range. A few remain, perhaps regularly, in coastal British Columbia, for Theed Pearse has given me five December dates and four February dates, spread out over a period of 10 years, on which he has recorded one or more Audubon’s warblers on Vancouver Island; on one of these dates, February 10, 1943, the temperature dropped to -6° F.
Rathbun tells me it is “of frequent, if not regular, occurrence during the winter” in Washington. And in Oregon Gabrielson and Jewett (1940) record it as a “permanent resident that has been noted in every county during summer and throughout western Oregon in winter. * * * Its little song is heard on every side during May and June, and its peculiarly distinct call or alarm note is a familiar sound throughout the balance of the year. This is true not only of the wooded slopes and bottoms but equally so of the weedy fence rows of the Willamette Valley, where during the short days of fall and winter these warblers may be found associating with the Golden-crowned Sparrows and Willow Goldfinches or sitting on the telephone wires with the Western Bluebirds.” Swarth (1926) writes:
In much of the West, especially in the Southwest, the Audubon’s warbler is one of the dominant species during the winter months. In southern California it vies with the Intermediate Sparrow and House Finch in point of numbers. Wherever there are birds at all, this bird is sure to be there. From the seacoast to the mountains, in city parks and gardens, in orchards and in chaparral, the Audubon’s warbler is equally at home. On any country walk scores are sure to be seen, starting up from the ground or out of the trees with wavering and erratic flight, showing in departure a flash of white-marked tail-feathers and a gleaming yellow rump spot, and uttering the incessant chip that, better than any marking, serves to identify the fleeting bird.
In colder sections there are some fatalities; in the Fresno district, according to Tyler (1913), “a period of two or three unusually cold nights frequently results disastrously for these little warblers, and my observations show that there is a greater mortality among this species than in all other birds combined. After a hard freeze it is not an uncommon occurrence to see certain individuals that appear so benumbed as to be almost unable to fly, and not a few dead birds have been been found under trees along the streets.”
From much farther south, in Central America, Dr. Alexander F. Skutch (MS.) writes:
“Audubon’s warbler is a moderately abundant winter resident in the higher mountains of Guatemala, yet like the closely related myrtle warbler, appears to be less regular in its time of arrival and departure and less uniformly distributed, than the majority of the more common winter visitants. These attractive warblers were abundant on the Sierra de Tecpán from January until April, 1933; but strangely enough they did not return in August or September with all the other warblers that winter there; and none had appeared by the end of the year, although I kept close watch for them. Yet in the middle of the following September, I found them numerous among the pine and alder trees on the Sierra Cuchumatanes, nearly 11,000 feet above sea-level. The males were then resplendent in their full nuptial dress of yellow, black, white and gray, and sang enchantingly. I believe it not impossible that they breed in this remote, little-known region—for here also I found a breeding representative of the savannah sparrow, hitherto known only as a migrant in the country—and it is to be hoped that some day an ornithologist will study the bird-life of this lofty plateau during the breeding season, from April to August.
“During the winter months, the Audubon warblers are truly social, and are nearly always met in flocks, sometimes containing 25 or more individuals. They are versatile in their modes of finding food. Sometimes, from the tops of the tall cypress trees near the summit of the Sierra de Tecpán, they would launch themselves on long and skillfully executed sallies to snatch up insects on the wing. As they twisted about in the air, they would spread their tails to reveal the prettily contrasting areas of black and white. At other times they foraged on the ground, like the myrtle warblers; and this habit brought them into contact with the bluebirds (Sialia sialis guatemalae), which are likewise arboreal birds that frequently descend to hunt on the ground. At altitudes of 8,000 to 9,000 feet I almost always found the Audubon warblers and the bluebirds together in the bare, close-cropped pastures where there were scattered, low, oak trees; and this association was so constant that it could not have been accidental. Both kinds of birds were exceedingly wary as they hunted over the ground, and would fly up into the trees if they espied a man approaching them, even from a long way off. The Audubon warblers, probably because they more frequently enter open, exposed places, where they are conspicuous and far from shelter and must exercise great caution not to be surprised, were by far the shiest and most difficult to approach of all the warblers of the Sierra, whether resident or migratory. This was true whether they happened to be in the trees or on the ground.