Warbler in the willows

By BILL CHAISSON
Of A Feather
The yellow warbler is one of the most widespread wood warblers. It breeds from Alaska to Labrador and south to Baja California in the west and South Carolina in the East. It is one of the warblers that does not actually live in the woods. It favors edges, hedgerows, and the scrubby stuff along streams and ponds, particularly if the shrubs are some sort of willow.

As is not unusual with a widespread species, it displays much regional variation in its appearance. It is not quite settled as to whether Setophaga petechia is conspecific with two tropical populations, the mangrove warbler (S. p. erithachorides) and the golden warbler (S. p. petechia). But even within the various petechia populations there is variation in the amount of chestnut streaking the males (in particular) have on their breasts, sides, and flanks.

This year the habitat around my home has succeeded to a seral stage that the yellow warbler finds appealing. The “Atlas of Breeding Birds in New Hampshire” notes that this species is usually found below 1,000 feet elevation and we are ~925 feet. So, toward the edge of their tolerance for what? Cold nights? In any case, the saplings and shrubs along the brook that flows through the property seem to have reached a height and density deemed acceptable to at least one pair of yellow warblers. I regularly hear and occasionally see one of them on my morning walks.

According to the “Atlas,” I will not be seeing them for long. Most New Hampshire birds begin migrating south in August. This reference also mentions that they stop singing “early in the breeding season.” I do not hear the local male every day anymore, but he still sings.

My venerable (1917) “Birds of America” volume supplies regional vernacular names for each species and the yellow warbler was also called the “summer warbler”; “summer titmouse”; “summer yellowbird” (among other names), perhaps in recognition of its short stay here in the temperate zone. It is also, like the goldfinch, occasionally called the “wild canary.”

Another bird I occasionally hear and see in my neighborhood is the brown-headed cowbird. The yellow warbler is a frequent victim of cowbird nest parasitism, but also one of the only species to regularly successfully resist it. I have seen photographs of warbler nests with three or four levels, each occupied by a cowbird egg that the warbler has covered by building a new nest over it. In 1917, George Gladden, writing “Birds of America,” called the yellow warbler’s unique cleverness “one of Nature’s riddles of which there appears to be no solution.”

In the century since then, much observation has been done. In 1955 in the journal Jack Pine Warbler, A.J. Berger reported a six-tiered nest. It “contained 11 cowbird eggs, distributed as follows: 1 cowbird egg within original base of nest (laid during nest construction), three cowbird eggs in first tier, one in second tier, two in third tier, two in fourth tier, one cowbird plus one warbler in fifth tier and one cowbird in top; nest was 14.6 cm tall.” In a 1987 publication for the Royal Ontario Museum, G. K. Peck and R. D. James inspected 399 parasitized nests in Ontario and recorded 29 two-tiered nests, seven three-tiered, two four-tiered, and one five-tiered nest.

In a 1989 paper in Animal Behavior, M.C.J. Burgham and J. Picman noted that first-year females merely uttered an alarm call at the approach of a cowbird, but older females remained on the nest and spread their wings over it protectively. If the cowbird approaches before the warbler laid any eggs or early in the laying sequence (yellow warblers lay up to six, one a day), the female warbler is more likely to abandon the nest and build another.

Even if the warbler manages to defeat the cowbirds, it will have laid fewer eggs, thus reducing its reproductive success. Observers have found that in some cases some of the warbler nestlings survive being raised alongside a cowbird.

Alexander Sprunt, Jr., writing in “The Warblers of America” (1957) claimed this species was “very confiding and gentle in its behavior, it will sometimes alight on one’s hand and feed its young if held there.”

Sprunt supplies some details about the differing appearance of various populations around North America as they were conceived in the late 1950s. The “type locality” (where the species was first described) is Quebec City. In the males of this population the “forepart of the crown [is] yellow tinged with orange, rest of upper parts olive green; sides of head and underparts bright yellow, streaked with chestnut; wings and tail edged with yellowish-green.” This subspecies aestiva is found from the Yukon and Ontario east to Nova Scotia and south to Nevada and New Mexico and east to northern Alabama. It is therefore our breeding yellow warbler.

The “Newfoundland yellow warbler” (amnicola) breeds north of the type subspecies, from Alaska to Newfoundland, south to British Columbia in the west. It has duller yellow underparts and heavier streaking.

The “Alaska yellow warbler” (rubiginosa) breeds along the coast from southern Alaska to Vancouver Island. It is smaller and more dully colored than aestiva.

The “Rocky Mountain yellow warbler” (morcomi) breeds between the Rockies and the basin and range terrain of Nevada. Its upperparts are darker with less yellowish green.

The “Sonora yellow warbler” (sonorana) lives in the eponymous desert, and it is the palest of all the North American subspecies.

In the modern classification the above five constitute the “northern” or aestiva group.

Finally, Sprunt recognizes the two subspecies that are still accepted today: the golden warbler of Caribbean and Central America and the mangrove warbler of Baja California and through Mexico to Guatemala. These have since been designated “groups” and split into an additional 40 (!) subspecies with additional monotypic Galapagos group broken out for good measure. Many of the golden warbler subspecies are confined to single islands.

Bill Chaisson has been a birdwatcher for over 50 years. He lives and works in Wilmot.