Summer Treefrogs and Cricket Frogs

This post serves as a follow-up to my earlier remarks about chorus frogs that breed in the late winter and early spring months.  By now, we are well into summer, which means that the spring peepers and upland and Cajun chorus frogs are silent, but other species of frogs are extremely vocal.  Mississippi is home to quite a few taxonomic families of frogs and toads, including Ranidae, or the true frogs (such as bullfrogs and leopard frogs), individuals of which are largely aquatic and can be found in ponds, lakes, and streams; Bufonidae, or the true toads; Microhylidae, or the narrow-mouthed toads; Pelobatidae, or the spadefoot toads; and Hylidae, or the treefrogs, a group that encompasses the chorus frogs (including the spring peeper and the upland and Cajun chorus frogs), as well as many other species that can be heard calling right now.  Let’s take a look at some of these hylids.

Treefrogs are distinguished by the sticky pads at the ends of their toes, which enable them to climb.  They are typically found in trees and shrubs near sources of water.  In the southeastern U.S., one of the most common species is the American green treefrog (Hyla cinerea).  As its name suggests, this frog is typically bright green.  However, like other treefrogs, it is capable of changing color to adapt to changes in temperature and environmental surroundings.  It usually, but not always, has pale stripes running along its sides, and its skin is very smooth.  The call is a nasally quank that carries for a good distance.  Large choruses of this frog can be heard in almost any wet, wooded area.

Another common species is the squirrel treefrog (Hyla squirella), a small frog with a raspy call that is reminiscent of a squirrel’s barking sound.  This extremely variable species may appear almost any color from bright green to dusky brown, depending on environmental conditions.  At first glance, a bright green specimen of the squirrel treefrog might be mistaken for a green treefrog.  However, the squirrel treefrog is smaller and always lacks the pale lateral stripes of the green treefrog.  Squirrel treefrogs were formerly restricted to the southern half of Mississippi, but have recently expanded their range northward. 

The next three species to be discussed, the common gray, Cope’s gray, and bird-voiced treefrogs, can be visually separated from the green and squirrel treefrogs by virtue of their bumpy skin.  Like all treefrogs, they are more likely to be heard than seen, so it is still a good idea to learn to recognize them by their calls.

As their name suggests, gray treefrogs are frequently mottled gray in color, although they may also be various shades of green or brown—again, depending on temperature and habitat.  The two species of gray treefrogs, common gray (Hyla versicolor) and Cope’s gray (Hyla chrysoscelis), look identical and are impossible to distinguish from each other in the hand.  This is due to the fact that one species, the common gray treefrog, is thought to have diverged from the other, the Cope’s gray treefrog.  Interestingly, common gray treefrogs are tetraploid, meaning that they have four pairs of chromosomes (i.e., genetic material)—twice the usual number.  Tetraploidy is fairly common in plants and invertebrate animals, but it is not as frequently observed in vertebrates.  While common gray and Cope’s gray treefrogs cannot be reliably distinguished from each other by sight, they can be easily identified by sound.  Both species have low-pitched trills, but the Cope’s gray treefrog’s call is faster and more snore-like than the drawn-out trilling of the common gray treefrog.  If you are a Mississippian, though, it's possible that you will not need to distinguish between the two, since the common gray treefrog may occur in only the northernmost sliver of the state: http://maps.iucnredlist.org/map.html?id=55687.  The Cope's gray occurs statewide: http://maps.iucnredlist.org/map.html?id=55448.

Bird-voiced treefrogs (Hyla avivoca) look like smaller versions of gray treefrogs.  One quick way to identify them in the field is to look at the ventral portions of their legs; unlike gray treefrogs, which have bright orange skin in this area, bird-voiced treefrogs have greenish-yellow.  You should have no trouble with identification if you hear the frogs, though; the whistling, bird-like call of the bird-voiced treefrog is drastically different from the sounds of gray treefrogs.  Also, while you could find gray treefrogs in almost any type of forest, bird-voiced treefrogs tend to be most common in swampy woods, especially those containing cypress and tupelo trees.  This is one of my favorite frog species in the state.  

The pine woods treefrog (Hyla femoralis) and the barking treefrog (Hyla gratiosa) are two species found in the deep southeastern portions of the U.S.  Unfortunately, I have had few encounters with either, and have heard pine woods treefrogs only in forests on the coast.  However, a friend and mentor of mine has heard that species as far north as Newton County, Mississippi, so it is certainly possible to find them fairly far inland.  They have strange calls—perhaps best described as sounding like hyperactive Morse code messaging.  The call of the barking treefrog, on the other hand, is throatier in sound than the green treefrog’s call.  Choruses of this species bring to mind—well, for me, anyway—hundreds of bouncing rubber balls.  The barking treefrog frequently has dark spots and is also noticeably bumpy-skinned, making it easy to distinguish from green treefrogs.  In the hand, pine woods treefrogs could be mistaken for squirrel treefrogs, but their inner thighs have rows of pale spots, unlike those of squirrel treefrogs.

The tiny, aptly-named cricket frogs are not technically treefrogs, as they belong to the genus Acris.  Nevertheless, they are hylids, and, like their relatives, they can commonly be found in vegetation near water.  Unlike treefrogs, cricket frogs tend to stay very low to the ground.  The southern cricket frog (Acris gryllus) and the northern cricket frog (Acris crepitans) are the two most common species in the state.  They are similar in appearance, and their insect-like calls are also a bit difficult to distinguish from each other.  However, the southern cricket frog has a raspier sound than the northern.  Both of these tiny frogs can be found in grasses and herbaceous vegetation near water.

So much for the adult hylids—but what about the earlier life stages: eggs and larvae?  Well, if you peer into small ponds, woodland pools, roadside ditches, or potholes on old dirt roads, you might be lucky enough to see these.  Frog eggs are in clusters, while toad eggs are in strings.  Tadpoles may take a month or more to fully metamorphose.  May the summer months be hopping!

Sources

The IUCN Red List of Threatened Species, http://www.iucnredlist.org/

USGS Frog Quiz, https://www.pwrc.usgs.gov/frogquiz/index.cfm?fuseaction=main.lookup

Species Profile: Cope’s Gray Treefrog (Hyla chrysoscelis), Savannah River Ecology Laboratory, http://srelherp.uga.edu/anurans/hylchr.htm

Species Profile: Pine Woods Treefrog (Hyla femoralis), Savannah River Ecology Laboratory, http://srelherp.uga.edu/anurans/hylfem.htm

 

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.