Bird of the Month - September 2004
Chestnut-sided Warbler
Dendroica pennsylvanica
It’s late morning at the ‘Horse Pasture’ willow patch at Fairhaven, the marine layer breaking up into tendrils and dissipating before the first onshore breezes of the day. Although it’s only the second week in September, the leaves of willows have begun to fall. The call-notes of Black-capped Chickadees and the chip calls of warblers betray a flock of songbirds somewhere up ahead. Birds silhouetted and but half-observed jitter and fly-catch in the willow canopy, each begging the application of binoculars and patience. Multiple Wilson’s and Orange-crowned warblers are seen, along with two or three Warbling Vireos. Yet another small bird draws the eye, and suddenly here is no Orange-crown! It has pale gray underparts, yellowish wingbars, a crisp circular eye-ring, and a remarkable deep yellow crown. It’s a bright first-fall Chestnut-sided Warbler, member of a species which doesn’t breed anywhere near California, but appears rarely in migration.
Chestnut-sided Warbler is one among a large group of warblers we tend to consider ‘Eastern’, even though the breeding range of this species and others extends into Alberta. It occurs in summer across much of southern Canada and in the northeastern United States. Nearly all Chestnut-sideds normally migrate, nest, and spend the winter far from California. Yet during periods of migration, a very few stray well to the west of this range. It is during late May and early June and again in September and early October that birders in the Redwood Region enjoy a chance to encounter one of these beautiful creatures. This species is not at the top of the list of the most frequently detected Eastern warblers, but it is in the top half-dozen. During the past decade, a half-dozen Chestnut-sided Warblers has been the annual average in Humboldt County, with most of these found during the fall migration. Nearly every one has been either along the coast or in the near-coastal lowlands. These birds have also made many appearances elsewhere along the northern California coast, with far fewer occurrences yet documented along the southern Oregon coast.
Searching for ‘vagrant’ warblers from east of the Rockies is a pastime a little like casino gambling. With big payoffs infrequent, you’d better enjoy the action. And the ‘action’ pays off through mornings or after-work junkets targeting one or two prime willow thickets with a history of rare warblers. Absent perfect weather conditions or an amazing wave of rarities, the odds seldom favor success in simply going out and finding a vagrant. A friend of mine (who might have headed the Department of Redundancy Department) once said, “Rare birds are rare because they’re rare.” But what nicer way is there to better understand the routine ebb-and-flow of the migration of Orange-crowned and Wilson’s warblers and Warbling Vireos? Ninety-nine percent of birding is experiencing common birds in common places.
Chestnut-sided Warblers in spring are vividly patterned with a yellow crown, black facial bridle, and rich chestnut-brown flanks. Males are more boldly patterned than females. Many spring vagrants are initially detected by their loud, accented songs. In fall, nearly all of the birds occurring on the West Coast and at desert oases are juveniles. These are known by pale gray underparts and face, neat white eye-rings, intense chartreuse-yellow crowns, and off-white to yellowish wingbars. All Chestnut-sided Warblers are somewhat chunky, and tend to cock their tail above the level of the wing-tips while foraging, useful clues in the low light prevailing in the thickets they favor.