Focus on Worm-eating Warbler

By dgross July 21, 2013

Worm-eating Warbler, commonly found in oak forests with Scarlet Tanager, by Gerry Dewaghe

The Worm-eating Warbler is not one of our most charismatic forest birds, but it is an interesting one. Its conservation issues were recently featured by the American Bird Conservancy, which named it the Bird of the Week. Pennsylvania’s forests are very important for the future of this curious songbird, and any spring or summer walk along a ridgeline forest could yield a few of them if the observer knows what to listen or look for.  Penn’s Woods may account for 10 percent of the total breeding population of the Worm-eating Warbler. This is one of the species that certainly is getting counted in the “Birding the Ridge” project (see a previous PA eBird story).

One of our most poorly named birds, the Worm-eating Warbler is a characteristic songbird of our mixed and deciduous forests. Its name is misleading as this warbler forages primarily for caterpillars, largely the larvae of moths, which once were commonly misnamed “worms.” The Worm-eating Warbler is a small wood warbler of the forest understory. Often overlooked, the unobtrusive Worm-eating Warbler spends most of its time in shrubby vegetation close to the ground, usually dwelling within a few feet of the forest floor. This bird may be heard more often than seen, though its song, too, may go unnoticed or mistaken for that of a trilling insect or a more common bird, especially the omnipresent Chipping Sparrow. It also has a curious foraging behavior called “dead-leafing” where it probes dead leaves and leaf clusters with its long straight bill. It forages like this on both its nesting grounds and on its wintering grounds in Central America. You could say it “hangs out” in our high quality forests.

The Worm-eating Warbler is olive-brown at a glance with a buffy head and underparts, and distinct black stripes on its crown and through its eyes. These bold head stripes are helpful field marks. Its legs are noticeably pink.

The Appalachian Mountains are at the core of the Worm-eating Warbler’s breeding range. Pennsylvania accounts for approximately 10 percent of the total nesting population, so it is critical that our state maintains the quality and quantity of forest needed for the continued existence of this songbird. The northern edge of the Worm-eating Warbler’s breeding range extends through Pennsylvania following the eastern edge of the Appalachian Mountains. It breeds primarily east of the Allegheny Front and is much less common, even accidental in some counties, in the western and northern parts of the state and absent through the northwest corner in summer.

This warbler is strongly associated with Pennsylvania’s deciduous forests during the breeding season and migration. It is a forest interior nesting bird. The Worm-eating Warbler inhabits steep wooded hillsides, slopes and ravines with patches of dense understory including wooded hillsides thick with viburnum, rhododendron, or mountain laurel. It is also found in swampy woodland when thick layers of small trees and shrubs are present. Worm-eating Warblers can be fairly common, although overlooked, in hillside deciduous forests including game lands.

In the forest understory, the Worm-eating Warbler specializes in recovering invertebrates from suspended tangles of dead leaves. This warbler seeks out dangling leaf clusters and pries or tears each crisp leaf open with its slim pointed beak to expose caterpillars, spiders and small insects. It will systematically work its way up through brambles and vines searching through the hanging curled leaves for its meal. The Worm-eating Warbler will also glean insects off green leaves and probe the bark of tree trunks and branches for insects.

Worm-eating Warblers inhabit low bushes and shrubs nearly all of the time, with the exception of males on territory. Like many species, male Worm-eating Warblers sing from high points, often a mid-level perch, to establish and protect breeding territory and to attract a mate. The Worm-eating Warbler’s song is a very dry, insect-like, trill like a shortened version of a Chipping Sparrow’s song but deep in the woods. Its song is generally less than two seconds long while a Chipping Sparrow’s song is usually more than two seconds. This bird can be fairly common in dry oak woods with a good understory of shrubs and saplings. In Pennsylvania, the Worm-eating Warbler may arrive back on breeding territory in late April; however, the peak of their return falls in the first two weeks of May.

This wood warbler nests on the ground, typically at the base of a sapling and often on a slope near water. Against the trunk of a young deciduous tree, the female builds an open cup nest of leaves, and lines it with moss and grass. While on eggs, usually three to six, females blend well with the surrounding leaf litter. The incubating female relies on its camouflage and sits tight on the eggs. If approached, the bird will flush¾usually at the very last second and sometimes only after contact. Once flushed the female attempts to distract the intruder with commotion bluff routines. Although naked and helpless upon hatching, young Worm-eating Warblers leave the nest within 10 days. This ground nesting warbler is particularly vulnerable to nest predators like chipmunks, shrews, squirrels and other small mammals.

As early as late July, the Worm-eating Warbler may depart from breeding grounds and head south. By the second or third week of August migration is well under way. This neotropical migratory bird will travel to mature tropical forests in Mexico, Central America and the Caribbean. It arrives on wintering grounds by October. This warbler relies on primary or native forests and undisturbed secondary forests with thick understory, in these distant places during winter. On its wintering grounds, the Worm-eating Warbler fills a niche by concentrating on its unique feeding behavior of ripping and probing dangling leaf clusters from the bottom up for invertebrates, almost like a miniature parrot.

As a forest interior species the Worm-eating Warbler depends on healthy contiguous forest throughout its breeding grounds in Pennsylvania and eastern United States and is vulnerable to forest fragmentation. Like other ground nesting forest birds, loss of good quality forest habitat may lead to population declines because of threats to nesting success due to predators that thrive in forest edge habitats such as domestic cats, crows, Blue Jays, Common Grackles, raccoons, opossums and black rat snakes. In a fragmented forest this warbler is also vulnerable to the Brown-headed Cowbird, a brood parasite which benefits from forest fragmentation.

Habitat decline on migration stopover sites places additional survival challenges on the Worm-eating Warbler and other migratory birds, especially along the shores of the Gulf of Mexico where many warblers stage before flying south to the Yucatan Peninsula and Central America. Possibly the greatest threat lies on its wintering grounds in Mexico, Central America and the Caribbean where deforestation of broadleaf forest continues to reduce forest habitat at alarming rates.

Your eBird trip data anywhere on the Worm-eating Warbler’s journey — wherever it “hangs out” —- is a valuable contribution to learning more about this obscure but fascinating little forest songbird.

By Kathy Korber and Doug Gross

Thanks very much to Gerry  Dewaghe for permission to use his marvelous photograph of a Worm-eating Warbler.