The Swainson's Warbler

My first “sighting” of a Swainson’s Warbler wasn’t really a sighting at all.  I was birding one of the local sewage treatment ponds with my then-mentor, and I heard an unusual song coming from the woods behind us.  My birding mentor identified it, but to try to find the singer in the dense, swampy woods would have been futile.  I didn’t encounter another Swainson’s Warbler until the fall of that year, when one showed up in a thicket behind my house, walking around in the leaves less than five feet in front of me.  After this, I was able to hear, and occasionally glimpse, Swainson’s Warblers many times during the spring and summer.  This elusive bird, once practically unknown to me, was present in a surprising number of places.

The Swainson’s Warbler is very local and difficult to see but is not an exceptionally rare bird over much of its breeding range in the southern United States.  It does have the distinction of being elusive enough to escape detection by ornithologists searching for it for decades.  Two of the most famous cases of “lost” avian species, the Ivory-billed Woodpecker and the Bachman’s Warbler, also had a history of being spotted once, then suddenly disappearing, not to be seen again for years.  These species are legendary for their secretiveness, but the Swainson’s Warbler also had a long history of disappearance and then rediscovery, before regular studies confirmed many facts about its once little-known lifestyle. 

The drab appearance of this diminutive creature seems to point to one reason it was overlooked for so long.  At first glance, the Swainson’s Warbler does seem unremarkable, especially when compared with other North American wood-warblers.  A small bird, but large for a warbler, the Swainson’s Warbler measures about five and a half inches from beak tip to the end of its short, slightly notched tail.  The bill, in fact, is different from that of most other warblers in being relatively long and sharply pointed.  The bird’s upperparts are grayish-brown, a dead-leaf color that serves as an effective camouflage.  As counter shading, the underside of the bird is a pale gray or whitish shade, but some individuals show a tinge of yellow in this area.  The crown of the Swainson’s Warbler is a warm shade of brown, making it look nearly chestnut next to the extremely drab upperparts.  The bird also has a faint eye-line on the grayish face, and its head is very flat at the forehead.  The warbler’s long, pink legs provide a clue to its lifestyle; it spends much of its time walking on the ground—an unusual behavior for a warbler—as it forages for invertebrates in thick leaf litter.

These features reveal much about the habitat of the Swainson’s Warbler, and in fact the real difficulty ornithologists had in studying this bird was the impenetrability of this habitat.  The Swainson’s Warbler prefers dense, swampy woodlands with thick undergrowth and a sparse ground cover, leaving the forest floor open to falling leaves and creating the slowly-decaying microhabitat that many species of invertebrates use.  Although it is not absolutely necessary, cane is often a component of this habitat; where stands of cane are present, the warblers build their nests in them near the ground.  Other stiff understory plants such as palmetto are, of course, also used to support nests.  In the northern areas of its range, parts of the Appalachian Mountains, the Swainson’s Warbler retreats into a very different habitat: mountain laurel and rhododendron are the usual understory plants in these ecosystems. 

Regardless of the plants making up the forest undergrowth in Swainson’s Warbler territory, the bird is extremely difficult to locate.  One of the few clues to its presence is its song, a loud, clear Whew whew whew whip-poor-will that falls in pitch, rising again at the last note.  The song is similar to that of the Louisiana Waterthrush, but the waterthrush’s song ends with a jumble of short trills and sharp notes, rather than the clear, ringing notes at the end of the Swainson’s Warbler’s.  The song may be loud for a tiny bird, but the singer is extremely hard to see.  It nearly always chooses a secluded perch somewhere in the upper mid-story level of the woods.  If the bird senses that it has been detected, it flies quickly to a new perch and sits there silently for several minutes until “danger” has passed, often to the chagrin of a birder wishing to see it!  I once tried to creep up on a singing Swainson’s Warbler, and the best view I got was of the bird’s tail end as the bird—having detected me—darted into a cane thicket.  But during migration, even normally shy birds like the Swainson's Warbler can be seen feeding out in the open, far from dense cover, when songbirds of all species engage in atypical behavior in order to exploit more food sources.

Swainson's Warbler -- pen and ink

If the Swainson’s Warbler is easy to overlook now, over a century ago it was barely known to exist.  The habits of the Swainson’s Warbler helped to keep its most basic life functions secret for decades; discovered by Alexander Wilson in 1832 and described by John James Audubon in 1834, it was nearly unknown and wasn’t reported again until William Brewster and Arthur T. Wayne, two pioneering American ornithologists, found it in South Carolina in 1884.  Unfortunately, after this discovery, the species was again unstudied for many years.  Among other things, these naturalists documented its nest for the very first time.  Since the 1930s, however, when birding became popular, we have learned much about this warbler.

One of the crucial details of the bird's life history—its nesting behavior—was first documented by Brewster and Wayne.  The female Swainson’s Warbler constructs the nest, a somewhat messy, bulky cup composed of twigs, leaves, and vine tendrils.  It is lined with a wide variety of materials, including grass, mosses, bark, and hair, and is usually built only a few feet up in the vegetation.  The eggs are an unmarked white—probably an adaptation to the secluded nature of the nest site.  Most birds with well-concealed nests have pale eggs, since the eggs themselves don’t need camouflage.  This is very common in cavity-nesting birds, and the nest of the Swainson’s Warbler is so well hidden that it almost functions as a cavity—it could pass for just a clump of dead leaves if seen by a casual observer.  The female seldom leaves the nest during the incubation period; she alone hatches the eggs, which takes 13 to 15 days.  Both parents tend the helpless, altricial young until they fledge, ten to twelve days after hatching.  Apparently the chicks are fed on the same kinds of insect larvae, spiders, and millipedes that constitute the adults’ diet.  The long, pointed beaks of the warblers enable them to probe the crevices of dead leaves and even the top layer of forest soil for their prey.  As they shuffle around on the forest floor, they will even flip dead leaves over with their bills to inspect the undersides, causing a slight rustling sound similar to that made by other southeastern forest species, such as the Eastern Towhee. 

Although the Swainson’s Warbler is an uncommon and fairly local species in most of its range, exceptionally good breeding habitat can support far more than the usual number of nesting pairs; up to 44 singing males have been reported in some areas.  The Swainson’s Warbler is, unfortunately, facing many of the same problems that are contributing to the decline of other Neotropical birds.  Because it generally prefers dense woodlands with canebrakes, it is losing habitat on both its breeding grounds and its wintering grounds in the Bahamas, Cuba, Jamaica, Mexico, and Belize.  The unique Appalachian population, which some scientists believe to be a distinct subspecies, has declined precipitously.  In areas where its habitat is badly fragmented, the Swainson’s Warbler is also susceptible to brood parasitism by cowbirds.  Due to these factors, the National Audubon Society has placed the Swainson’s Warbler on its WatchList as a species of global conservation concern.

It’s amazing to think of the millions of years that helped shape the unique behaviors and adaptations of this unusual little bird.  Though we now know a great deal about its habits and place in the ecology of the forest, it took well over a century to discover them, and there are still secrets that it has yet to relinquish. The mysteries behind animals such as the Swainson’s Warbler are constant reminders of how much more there is still to learn about the natural world around us.

References:

Audubon WatchList 2007, National Audubon Society.  http://www.audubon.org/news/audubon-watchlist-2007.

A Field Guide to Warblers of North America.  Dunn, John, and Garrett, Kimball.  Houghton Mifflin, Boston and New York.  1997.

A Field Guide to the Birds’ Nests.  Harrison, Hal H.  Houghton Mifflin, Boston and New York.  1975.

Birds of Mississippi.  Turcotte, William H., and Watts, David L.  University Press of Mississippi, Jackson.  Mississippi Department of Wildlife, Fisheries and Parks.  1999.

The Cornell Lab of Ornithology’s Guide to Birds of North America, version 3.  2001.