Converging on Thrushness

Of A Feather
By Bill Chaisson
I am intrigued by the phenomenon of convergence. Why would unrelated species look so similar? How similar are they, really? Obviously, members of a given population must be able to identify one another in order to breed. But the concept of convergence has been used broadly and narrowly. Broadly, we think of some marsupials as converging on the appearance of placental mammals. The extinct thylacine was variously called the Tasmanian tiger or Tasmanian wolf because it had adaptations that caused it to resemble those predators. Marsupials and placental mammals are in different infraclasses, that is, not at all closely related. But filling the same ecological niche caused the evolution of similar appearance.

Birds belong to the class Aves, which is divided into orders (without the intervening level of infraclass). The vultures offer a good example of convergence between orders: New-World vultures (Cathartiformes) are considered to be in an entirely different order from the Old-World vultures, which are most closely related to most other raptors (Accipitriformes). Initial DNA research put New-World vultures closer to the storks (Ciconiiformes), but mitochondrial DNA studies showed them to be a separate but “sister order” to the Accipitriformes. In spite of not being closely related, both groups of carrion-eating birds are called vultures.

Birds within an order can resemble one another enough that we give them the same common or vernacular name. As European naturalists traveled the world during the period of colonization (16th to 19th centuries) they encountered birds that resembled in appearance and behavior species they already knew. The world, for example, is apparently particularly full of small, energetic brown birds that carry their tails straight up over their backs. Hence scrubwrens, fairy wrens, grasswrens, emu-wrens, wren-babblers, and the wrenthrush. All of these are in different families from the Troglodytidae, the actual wrens.

I was reminded of this phenomenon during a bird walk last Saturday, when the leader looked at a stretch of rocky brook overhung with shrubs and said, “There ought to be waterthrush in there.” Within minutes we heard the distinctive song of the northern waterthrush (Parkesia noveboracensis), which, in spite of its name, is a member of the Parulidae—a New-World wood warbler—not the Turdidae (thrushes).

The waterthrushes have a brown back, streaked underparts and long, strong legs on which they move above on or near the ground, foraging for invertebrates. In other words, they look and act a heck of a lot

like a thrush in the genus Catharus or Hylocichla. The waterthrushes are 6 inches long and the smallest Catharus thrushes, the hermit and the Bicknell’s, are less than 7 inches long.

I say “waterthrushes” because there are two species. The Louisiana waterthrush (Parkesia motacilla) is, as its name suggests, at the northern edge of its range here in New Hampshire, but it does occur here. The two waterthrushes are not convergent on each other; they actually are closely related. For a long time, they were thought to be also related to the ovenbird (Seiurus aurocapillus), which was once called the golden-crowned thrush, because all three warbler species resemble one another. But this too turned out to be convergence.

P. noveboracensis appears to be slightly smaller than P. motacilla; they are about the same length overall, but the latter is longer winged. The streaking on the underparts of the northern waterthrush appears to be denser and extends up onto the throat.

The light areas between the streaks of Louisiana waterthrush are buffy on the sides toward the rear, but most of the breast and side feathers between the streaks are white. To make matters confusing, there are two morphs of the northern waterthrush. In one, the light areas of the underparts are suffused with a light yellow, and in the other, they are white.

The white superciliary streak (over the eye) is narrow in the northern waterthrush and widens toward the back of the head in the Louisiana waterthrush.

The best way to tell the species apart is by their songs. That of the northern waterthrush is loud, strident, and the notes are delivered in threes with each triplet descending in pitch and more hurried. In contrast, the Louisiana waterthrush’s song begins with two to four slurred notes on the same pitch followed by a jumble of notes at a lower pitch.

After hearing the northern waterthrush last Saturday, on Sunday we went over to Danbury to have lunch by the Smith River. A brown-backed bird with streaked underparts swooped across the clearing and alighted on a balsam sapling. The range of the northern waterthrush extends south from Alaska and northern Canada to New England and the northern Appalachians. The yellow morph is more common in the north and decreases southward. The bird I saw in Danbury was a whitish individual, but I guessed it was P. noveboracensis by the density of its streaking. Then it flew back into a swamp and began to sing and that confirmed the identification.

Two years ago, I went kayaking along the Lane River between Kezar Lake and the Sunapee-Ragged-Kearsarge Greenway, a place called Aquavilla Swamp. There were singing northern waterthrushes about every 50 yards along the river there. It was the third week of May and the shrubs and trees were not yet

leafed out, but these warblers had arrived to claim their territories. They may have been particularly vocal because there were so many of them in this ideal habitat; the probability of a neighbor making an incursion into one’s turf was unusually high.

The Catharus thrushes are known for their tail pumping—flicking it up and then slowly lowering it—when they are alert and nervous. The waterthrushes have carried the convergence beyond appearance into behavior; they are tail flickers as well. The Louisiana waterthrush can be told from the northern species because, according to Sibley, it “bobs its tail more slowly and in a semicircular fashion.”

Bill Chaisson has been a birdwatcher for over 50 years. He lives and works in Wilmot. Contact him at [email protected]