A rare southern warbler

By BILL CHAISSON
Of a Feather
Merlin occasionally has hallucinations, your first clue that it is an example of “artificial intelligence” creeping into our lives and the subject of such hype over the last year or two. It regularly mistakes the black-throated blue warbler for the cerulean, which makes me feel better about doing the same thing. But on a bird walk last weekend it threw up the “suggestion” that it had heard a Swainson’s warbler, a bird I don’t really know and have never seen.

It is a very southern bird, normally not found north of Maryland, but according to David Sibley’s guide, on rare occasions spotted in Nova Scotia and upstate New York. There is no entry for it at all in The Birds of New Hampshire, which means it was not seen here before 2013. Unnervingly, someone reported a Swainson’s warbler in northern Sutton in August 2022. I say “unnervingly” because that observation is less than two miles from where Merlin thought it heard one on May 18 in Wilmot. Hallucination … or not? I will have to go back and check. Not least because Merlin also thought it heard a cerulean there …

The Swainson’s warbler was first collected in 1833 by John Bachman of Charleston, S.C., who turned his specimen over to John James Audubon. In 1834 Audubon named it after William Swainson (who would eventually have a thrush and a hawk named for him as well, although not for long now). Its genus name, Limnothlypis, means “marsh finch,” and it was initially seen in marshes and bottomland forests. But it was seldom seen. It favors dense undergrowth, but not always wet habitat. It is also found in early succession stages of pine forest but also in the mixed mesophytic forests of the southern Appalachian Mountains.

It is thought to have never been common and its population is estimated at about 140,000 today, which is quite low for a warbler. The distribution of the Swainson’s warbler is very patchy and the main threat to it seems to be habitat destruction. It ranges from eastern Texas through Arkansas and east to Georgia. It follows the western slopes of the Appalachians north to southern West Virginia and the East Coast to southern Maryland. Bottomland forest over this whole region has been converted to agricultural use or flooded by reservoirs. The species is also associated with cane breaks and the giant cane has been reduced to 4% of its original range.

The Swainson’s is an unusual looking warbler in that it is not colorful: it is a light brown above and a buffy off-white below. No streaks. No wing bars. No spots. It has a dark line through the eye, a light line above the eye, and a rufous-tinged crown. The bill is heavy and sharp-looking for a warbler and the tail is quite short and broad. It somewhat resembles a Carolina wren. Sibley renders its song as “sew, sew, SISTerville with down slurred notes at the beginning an emphatic ending (compare to Louisiana waterthrush, hooded warbler).”

Sibley places it on the same page with the ovenbird and opposite the waterthrushes, as they are also brown and tend to stay close to the ground, like the Swainson’s. All three of those species, however, have strongly streaked undersides. Like the waterthrushes and ovenbird and unlike most other warblers, there is little or no difference in the appearance of the male and female birds.

My trusty Birds of America, although written in 1917, regards the bird as rare. Their description is quite elaborate, in large part because in those days they shot the birds and described them in hand. For example, the crown is said to have an “indistinct center streak of paler, or an indication of one.” The color of the back is said to be olive (not brown) and there is supposedly “a triangular spot of dusky in front of the eye.”

The nest is built 3 to 10 feet above the ground in cane or shrubs and is “a remarkably large affair of water-soaked sweet gum, water oak, pepperidge [black gum or tupelo] or holly leaves, lined with fine pine needles and moss.” In 1917 its habitat was already known to be oddly inexact. The nests are often found in swampy locations but also in upland areas.

William Brewster (for whom the warbler is named, although not for long) describes the Swainson’s song as “a performance so remarkable that it can scarcely fail to attract the dullest ear … It consists of a series of clear, ringing whistles, the first four uttered slowly and in the same key, the remaining five or six given more rapidly, and in an evenly descending scale, like those of the cañon wren … In general effect it recalls the song of the waterthrush … It is very loud, very rich, very beautiful, while it has an indescribably tender quality that thrills the senses after the sound has ceased.” Brewster goes on (and on) to note that the warbler does not sing very often, but once it starts, the song is repeated immediately and successively several times.

Song and Garden Birds of North America describes the song as whee whee whee toot-tut-say-bee-o, closer to Birds of America than to Sibley’s drier rendering. They also observe that it is delivered from a favorite perch or as it forages on the ground where it has an “easy, gliding walk.” In their painting the throat and undertail coverts are white. The lack of a consensus on its appearance and its song are signs of its elusive nature.

When I rechecked my recording from May 18, the Swainson’s warbler had disappeared from the list. This is another AI feature. If it heard only a fragment of the song and heard it only once, Merlin will “think better” of its identification and delete it from the list. In effect, it says “never mind.” The “cerulean warbler,” however, was still there.