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.
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| 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.
