Forestry for the Birds

This article was originally published in the Winter 2024 Issue of Illinois Forests

The cerulean warbler is a Nearctic-Neotropical migrant songbird that breeds in mature deciduous forests in the Midwest and eastern United States and migrates as far south as the northern Andes mountains in the winter. Cerulean warblers, and many other warbler species populations, have declined substantively starting in the 1900’s. This species is listed as a bird of conservation concern for the Midwest region by the U.S. Fish and Wildlife Service and numerous bird organizations, is listed as “threatened” in Illinois, and a Regional Forester’s Species of Concern on the Shawnee National Forest. The primary reason for their declines is the loss of large contiguous forested habitats on their summer and wintering grounds. Partners in Flight, an international bird conservation organization, estimates that cerulean warbler populations may decrease an additional 50 % within 25 years if no conservation measures occur to address habitat loss and degradation in its forested breeding and non-breeding habitat. 

Historic work has occurred on cerulean warblers in southern Illinois, however there are no updated or current databases with records that focus on this species and no extensive work has focused on the species in Illinois. In neighboring Indiana, Dr. Kamal Islam has mentored several students and worked on cerulean warblers on managed forests and published several papers detailing their habitats, management, and life histories. Knowledge of populations of cerulean warblers and their habitats in Illinois is critical for developing management to restore and maintain their preferred habitat and to understand their responses to forest management activities. 

To better understand current cerulean warbler populations and their summer homes on the Shawnee National Forest, we conducted a cooperative survey effort with the Illinois Nature Preserves Commission in likely habitats in May–July of 2021–2023. We documented 106 singing males on the forest from four counties. The greatest concentration, a total of 75 males, was found in the Cave Valley Bird Area of Shawnee National Forest. Understanding cerulean warbler habitat within the Cave Valley Bird Area is important to maintaining populations there but also using that information to develop forest management activities that could create, restore or enhance habitat for this species across the forest and its greater range. To collect these data we plan to utilize light detection and ranging (LiDAR), a remote-sensing technique. LiDAR analyses can measure tree canopy height, heterogeneity, and topography. Forestry professionals will also conduct timber stand examinations to measure basal area, tree species composition, and understory and midstory plant composition. Combining LiDAR with stand exam data collected from the ground will enable us to better understand existing cerulean warbler habitats and help develop forest management actions to restore and enhance habitat for cerulean warblers elsewhere. In sum, we now have the tools to hopefully decipher the summer home plans of the cerulean warbler in southern Illinois and on the Shawnee National Forest. 

We mostly observed cerulean warblers within the Cave Valley Area to be associated with large diameter, mature tall trees on slopes adjacent to the Cave Creek floodplain or along the Cave Creek or Cedar Creek. These characteristics align with habitat conditions described from other research efforts on cerulean warblers. Interestingly, some areas adjacent to and within the Cave Valley Bird TArea consist of non-native shortleaf pines and tulip poplar plantations, and other forested areas with little to no oaks competing for a place in the canopy. These habitats have proven not desirable for cerulean warblers and thus present a potential opportunity for habitat restoration or improvement. Creating a better mosaic of desirable tree species composition in the floodplain, particularly with more oaks, could be beneficial. Conducting forest stand improvements can enhance habitat by creating canopy heterogeneity and ensure future oak regeneration. In sum, cerulean warblers inhabit specific areas and it’s a good idea to maintain and improve areas around them for post-breeding adults and dispersing young. Maintenance of their summer homes is important but creating all-new summer homes from the ground up requires large contiguous forested tracts with the right ingredients and it would be a feather in our cap if it happened, especially in the Midwest. We think one of our on-going vegetation management projects, the Bean Ridge Project located in Alexander County IL, may hold promise for establishing at least some all-new summer homes for cerulean warblers. Developed to mitigate forest health issues and future risks associated with unhealthy forest conditions, the project consists of 2,656 acres of vegetation treatments ranging from first entry shelterwood treatments, thinning, midstory manipulations, and burning. Management activities like these that improve tree vigor by promoting larger mast producing trees, wider spacing, and improved canopy heterogeneity may provide habitat benefits to species like cerulean warblers who were absent during pre-harvest bird surveys. Improved forest conditions with steeper topography, coupled with the presence of white oaks and riparian areas make the Bean Ridge Project a potential site for attracting cerulean warblers post-harvest. The largest and most consistent increases in cerulean warblers in the Appalachian Mountains occurred when residual basal area was between ~40 and 90 ft2. Partial harvests like those being implemented in the Bean Ridge Project may help us further develop ideas and skills in decoding potential summer home plans for cerulean warblers on the Shawnee.

 Nonetheless, whether you are maintaining existing summer homes or trying to create some for cerulean warblers, active management not only benefits oak regeneration and healthier forests, it can also benefit other wildlife and recreation. Breaking up tree canopies, creating gaps, and lowering tree basal area can improve foraging and roosting habitat for federally listed bats. More sunlight to the ground may increase growth in the understory and provide better cover for white-tailed deer and nesting wild turkeys, and therefore, improve hunting opportunities. That’s good economics since the Shawnee is among the most important recreational sites for deer and turkey hunters in Illinois and for out-of-state hunters. In essence, managing forests for birds like the cerulean warbler can be beneficial to many users in the region and provide opportunities for humans and wildlife species that need to coexist.