Worm-eating Warbler Life History

Habitat

Worm-eating Warblers breed in deciduous and mixed forest interiors, especially on hillsides or slopes with abundant understory shrubs. They use forests with beech-maple, oak-hickory, hemlock, mountain laurel, and rhododendron in the northern part of their range. In the southern and coastal portions of their range, Worm-eating Warblers can also thrive in white-oak wetland forests with holly understory and in lowland bogs with sandy peat soil and woody shrubs. They have even nested in loblolly pine plantations. All these habitats have a lush understory and mostly closed canopies. More than most species, the Worm-eating Warbler appears to be sensitive to the fragmentation of breeding habitats; they need between 52 and 850 acres in order to nest. However, the species moves into some logged areas relatively soon (3 years) after tree removal. During migration, Worm-eating Warblers are most often found in similarly dense wooded habitats. In winter, in the Caribbean and Central America, they use mangrove forests, montane forests (pine, broadleaf, and mixed), evergreen rainforests, wet limestone forests, dry forests, and shade-coffee plantations.

Food

Worm-eating Warblers eat mostly caterpillars and grubs, not earthworms as their name might seem to suggest. They also consume slugs, spiders, insects, and larvae, which they hunt in clusters of dead leaves and in live foliage. Other prey items are sawfly larvae, leafhoppers, grasshoppers, beetles, walking sticks, and bugs, which they glean from vegetation and occasionally from leaf litter. In spring and early summer, they forage in leaf and flower buds, and flowers. They tend to forage in the forest interior, usually less than about 18 feet off the ground. In tropical forests, wintering birds capture insects in dead leaves and vine tangles; they also forage in loose bark and rotting wood. Worm-eating Warblers find most of their prey by gleaning (picking it from vegetation), sometimes by leaping or flying upward to capture insects. They often probe into bark or dead leaves when hanging upside down. Like starlings, meadowlarks, and other species with long, spikelike bills, Worm-eating Warblers “gape”—inserting the bill into a tight crevice, then opening the bill to better expose, and then seize, prey. On occasion, they pursue flying insects on the wing or hop in leaf litter, searching for slugs or beetles.

Nesting

Nest Placement

Worm-eating Warblers nest on the ground, usually on a sloped site, and often at the base of a shrub or sapling near water. Females select the nest site.

Nest Description

The female builds the nest; a cup of skeletonized dead leaves lined with stems of hairy-cap moss, hair, fine grasses, maple seed stems, and pine needles. The nest measures about 8 cm across.

Nesting Facts

Behavior

Worm-eating Warblers are largely monogamous, but some males have two female mates simultaneously (bigamy). The courtship display has not been described for this hard-to-see species, but threat displays include a “bow,” in which a territorial bird faces an intruder and leans forward, displaying its head pattern, and a “gape” display, in which the bird points its bill toward the intruder and opens it. Birds in conflict sometimes “freeze” in these positions. Males establish territories quickly after arrival in the nesting area in spring; territories cover about 4.3 acres, usually surrounded by extensive undisturbed habitat. Males drive other Worm-eating Warblers out of the territory and sometimes chase larger passerines such as Wood Thrushes. Females occasionally participate in fights and chases. Both adults tend the young, and males cease territorial behavior once the young have fledged. On the wintering grounds, the species is territorial but often joins foraging mixed-species flocks as they pass through the territory.

Conservation

Worm-eating Warbler populations have remained relatively stable since 1966, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 780,000 birds. Partners In Flight rates the species a 12 out of 20 on the Continental Concern Score, indicating a relatively low level of conservation concern. Use of insecticides to control Lymantria dispar (often known as gypsy moth) can cause declines in Worm-eating Warbler numbers. The species requires large forests for nesting, and the destruction of forest habitats represents a threat to its conservation. However, in some areas, Worm-eating Warblers appear to tolerate selective logging and even clearcutting. Worm-eating Warblers, like most songbirds that migrate nocturnally, often strike buildings and other structures and are killed. Feral and domestic cats probably kill these warblers during migration because they forage low in the vegetation, where cats often hunt.

Credits

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2019). The North American Breeding Bird Survey, Results and Analysis 1966–2019. Version 2.07.2019. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

Stephenson, T. and S. Whittle (2013). The Warbler Guide. Princeton University Press, New Jersey, USA.

Vitz, Andrew C., Lise A. Hanners and Stephen R. Patton. (2013). Worm-eating Warbler (Helmitheros vermivorum), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

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