The Birders’ Dozen Profile 12: Cerulean Warbler

By Dr. Jessica Outcalt

Welcome to the Birders’ Dozen! We’ve reached the last article in this series 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 Cerulean Warbler, named for the brilliant sky-blue color of the male, was once abundant in forests around river valleys in the eastern United States throughout the 1800s. In the latter half of the twentieth century, populations of Cerulean Warblers have decreased by as much as 3% per year, or 72% in total since the 1970s. Loss and fragmentation of mature deciduous forest have affected warbler populations, as they are dependent on large areas of forest with large trees and full canopies. This bird is a well-studied species and target of many conservation programs; in Indiana, it is listed as a state endangered species and identified as a species of concern by many groups due to population declines.

Natural History
This warbler is a mature forest specialist that forages on insects in the highest portions of the canopy as well as areas next to canopy gaps. Though sensitive to fragmentation of forests, possibly due to factors such as Brown-headed Cowbird parasitism and increased numbers of predators, Cerulean Warblers primarily use openings in upper forest canopies as their preferred breeding and foraging microhabitats.

White oak and bitternut hickory trees are preferred nesting and foraging trees, while red oak and red maple are generally avoided. While some plant matter may be eaten during migration, the Cerulean Warbler is almost exclusively an insectivore, gleaning insects such as caterpillars and aphids off the tips of branches. Because their primary food is insects, tree species such as white oak that support high numbers and diversities of native insects are important.

These bright blue warblers are generally monogamous, though males have been known to have multiple female partners within a breeding season. Nests are constructed out of plant fibers and spider or caterpillar silk, and are concealed in clumps of leaves or vines near forest canopy gaps. Though females are the primary nest-builders and sole incubators, males occasionally help and later will sometimes bring the female food while she incubates the clutch of 3-4 speckled eggs. These eggs are incubated around 11-12 days, and the young fledge around 10-11 days after hatching.

Cerulean Warblers are long-distance migrants, migrating through Central America and spending the winter in the Andes Mountains of South America. During non-breeding seasons, these warblers continue their pattern of using canopies rather than mid- or understories. This means they can often be found in habitats such as shade-grown coffee plantations, which leave the tropical forest canopy relatively intact while allowing important food crops to thrive.

 Cerulean Warbler, photo courtesy Matt Williams Nature Photography.

Habitat Management
Recommendations for silviculture that benefit Cerulean Warblers on breeding grounds include encouraging growth of large trees with full crowns, as well as a vertically diverse canopy. Since nests of Cerulean Warblers are found on limbs most often near canopy gaps or areas with dense foliage cover, uneven-aged management practices like single tree or group selection can create these necessary gaps. Even-aged management such as shelterwood harvests, if rotations are long, can also benefit Cerulean Warblers. Encouraging growth of oaks, especially white oak, and hickories can provide ideal nesting and foraging areas for warblers as well.

One of the primary concerns for Cerulean Warblers is the preservation of large, intact tracts of mature forest. This bird is area-sensitive, which means it needs large areas, at least 50 acres or more at a minimum. Preserving these large tracts of forest limits threats from concerns such as Brown-headed Cowbird parasitism and provides a stable habitat to support healthy breeding populations. Small disturbances that create canopy gaps are beneficial if not vital to Cerulean Warblers, but processes that create a large amount of edge habitat can be damaging to warbler habitat.

Conclusion
Brilliantly blue, high in the forest canopy, it’s easy to argue we’ve saved the prettiest bird for last in our series. These mature forest birds thrive in areas with a structurally diverse canopy, especially within oak and hickory forests. Cerulean Warblers also use canopies on their wintering grounds, and supporting agroforestry techniques that maintain a relatively intact canopy in Central and South America, such as shade-grown coffee, can further benefit these birds. We hope this series highlighting the birds from Forestry for the Birds’ Birders’ Dozen list has encouraged you that appropriate forest management can benefit both birds and people, and that management benefitting birds is possible on scales ranging from single trees in a backyard to entire landscapes.

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 is an independent consulting biologist who worked with The Nature Conservancy to develop the “Birders’ Dozen Profiles.” She is now a Natural Resources Training Specialist with Purdue University’s Department of Forestry and Natural Resources. 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

Bakermans, Marja H., Andrew C. Vitz, Amanda D. Rodewald, and Carlos G. Rengifo. 2009. “Migratory Songbird Use of Shade Coffee in the Venezuelan Andes with Implications for Conservation of Cerulean Warbler.” Biological Conservation 142 (11): 2476–83. https://doi.org/10.1016/j.biocon.2009.05.018.

Buehler, D. A., P. B. Hamel, and T. Boves (2020). Cerulean Warbler (Setophaga cerulea), 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.cerwar.01.

Jones, Jason, and Raleigh J. Robertson. 2001. “Territory and Nest-Site Selection of Cerulean Warblers in Eastern Ontario.” The Auk 118 (3): 727–35. https://doi.org/10.1093/auk/118.3.727.

Cerulean warbler. Partners in Flight. (2020, September 24). Retrieved March 2022, from https://partnersinflight.org/species/14179/