The Birders’ Dozen Profile 4: Worm-eating Warbler
Welcome to the Birders’ Dozen! Over the next several issues, we are going to continue introducing the bird species from Forestry for the Birds. The Birders’ Dozen are forest birds that can benefit from targeted management practices, as most are declining due to habitat loss. We’ve curated this list to cover a wide range of habitat types, from young to mature forest, open to closed canopy, or dense to non-existent shrub layers. Our goal is to engage landowners and foresters in the process of managing forests for wildlife, or “forests for the birds.”
The Worm-eating Warbler is the fourth bird in our list, and unlike last issue’s Baltimore Oriole, inhabits a much more specific habitat type with unique requirements. Its name comes from its food preferences for caterpillars, historically called “worms” – the American Woodcock from our second profile actually eats more earthworms than the Worm-eating Warbler does! This warbler breeds primarily in large forests of southern Indiana, one of the few species on our list that doesn’t regularly breed across the entire state.
Natural History
The Worm-eating Warbler is named for its habit of gleaning caterpillars (“worms”) from leaves, most often in the understory along slopes. Though its range does not extend across the entirety of the state, it is nonetheless an important Central Hardwoods species. Primarily considered a mature forest specialist, the Worm-eating Warbler also spends considerable time in dense, shrubby patches within forest landscapes.
The Worm-eating Warbler is found primarily where large areas of mature forest intersect with slopes and patchy shrubs. Sensitive to the overall area of a forest, these warblers nest on the ground hidden in low shrubs or leaf litter. Nests are often found near streams or wetlands, and in high densities within large forest tracts. Females build cup nests, and lay clutches of 4-6 small, speckled, and cream-colored eggs which hatch in just under two weeks. Nestlings fledge around 10 days after hatching.
Despite their pattern of mature forest utilization, habitat use during the post-fledging period (the time just after young birds leave the nest) in this species is unusual. In some areas, Worm-eating Warblers during the post-fledging period are among the most abundant birds in regenerating clearcuts. Short, dense vegetation, such as that found in clearcuts, is used in high numbers by these and other mature forest birds during this critical time, likely due to food availability and shelter from predators.
The Worm-eating Warbler migrates along the Gulf and East Coasts to its wintering grounds in the Caribbean and Central America, where it forages on insects and spiders. Fall migration generally peaks in late August or early September in Indiana, and spring migration peaks around early or mid-May. Dense shrub layers in the understory are important migratory habitat for these birds.
Habitat Management
Management for Worm-eating Warbler should be informed by both landscape- and nest-level characteristics. Large mature forests and presence of steep slopes are important landscape-level characteristics predicting warbler densities, and leaf-litter depth and shrub cover are important at microsite levels. In addition, presence of early successional, open areas near large tracts of mature forest can benefit warblers during the post-fledging and migratory periods.
On a large scale (hundreds of acres or more), Worm-eating Warblers depend on mature forest and steep slopes, which is part of the reason their range is restricted to southern Indiana. Ecologists often talk about “source” and “sink” habitats for wildlife, where large, high-quality habitats act as sources for population growth, but smaller low-quality areas cannot support growth. Large mature forest tracts can be sources for Worm-eating Warbler populations, areas that foster strong breeding populations.
Managers of large forest tracts should maintain healthy, mature forest with dense shrub patches to benefit warbler populations. Forest fragmentation is one of the primary threats to Worm-eating Warblers, so maintenance of connected forest is important for conservation of these charismatic birds. In particular, Worm-eating Warblers seem to use patches on southwest-facing slopes for nesting, so targeted management towards those areas can be beneficial.
On a smaller scale, such as management within your property, creating and maintaining habitat in which Worm-eating Warblers can breed will be beneficial. Dense shrub patches and a well-developed leaf litter layer, especially near oak stands, can be beneficial for nesting success. In addition, open areas such as those that result after clearcuts can be beneficial habitat during the post-fledging and migratory periods.
Conclusion
Though the Worm-eating Warbler’s habitat requirements seem daunting at first, targeted management for these charismatic little songbirds can benefit the entire forest ecosystem and provide breeding habitat for many other birds as well. Providing both mature forest tracts in areas with steeply sloped terrain for nesting habitat and young forest clearings for post-fledging and migratory habitat can greatly benefit declining populations of Worm-eating warblers, for the benefit of both birds and people.
Special thanks to the Alcoa Foundation, the Indiana Forestry Educational Foundation, and The Nature Conservancy for their support and leadership of Forestry for the Birds.
Jessica Outcalt, PhD was an independent consulting biologist who worked with The Nature Conservancy to develop the “Birders’ Dozen Profiles.” She is now an Agriculture and Natural Resources educator with Purdue Extension in Grant County. She completed her BS in biology at Taylor University, her PhD in wildlife ecology at Purdue University, and is passionate about birds and getting people involved in conservation and scientific processes.
References
Brock, Kenneth J. Brock’s Birds of Indiana – Worm-eating Warbler. Amos W. Butler Audubon Society, 2006.
Patrick J. Ruhl, Clayton D. Delancey, John B. Dunning “Roost preference, postfledging habitat use, and breeding phenology of adult female Worm-eating Warblers (Helmitheros vermivorum) on the breeding grounds,” The Wilson Journal of Ornithology, 130(2), 397-409, (1 June 2018) https://doi.org/10.1676/16-222.1.
Ruhl, P. J., K. F. Kellner, J. M. Pierce, J. K. Riegel, R. K. Swihart, M. R. Saunders, and J. B. Dunning, Jr. 2018. Characterization of Worm-eating Warbler (Helmitheros vermivorum) breeding habitat at the landscape level and nest scale. Avian Conservation and Ecology 13(1):11. https://doi.org/10.5751/ACE-01185-130111.
Vitz, A. C., L. A. Hanners, and S. R. Patton (2020). Worm-eating Warbler (Helmitheros vermivorum), version 1.0. In Birds of the World (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.woewar1.01
Worm-eating Warbler, photo courtesy Matt Williams Nature Photography.