Do Forest-dwelling Songbirds Experience Higher Rates of Cowbird Parasitism Immediately After Prescribed Fire?

The loss and fragmentation of natural habitats, including forests, are leading factors in the planet-wide loss of biodiversity, species extinctions, and population declines in the fauna that depend on those habitats. Over the past three decades, several studies in Illinois have found that songbirds nesting in highly fragmented forests suffer from low rates of nesting success because of the one-two punch of nest predators and brown-headed cowbirds (cowbirds hereafter). The mosaic of forest and agricultural/developed land tends to support high abundances of nest predators and cowbirds. As a result, in many forests within Illinois, songbird nests are likely to receive cowbirds eggs (which reduce the number of songbirds produced per nest) or fail because of nest predators (mammals, snakes, or some avian predators find nests and eat all the eggs or nestlings). Consistently high rates of nest predation and cowbird parasitism do not allow bird populations to thrive. This has led to a call for maintaining/conserving any large areas of forest that remain, and to defragment forested landscapes where there is potential to create forest interior by acquiring/afforesting relatively small amounts of land.

Illinois forests also face challenges associated with mesophication, a process that can shift forests away from oak-hickory dominance to dominance by shade-tolerant tree species like maples. Land managers are using various types of forest management tools (e.g., prescribed fire and targeted thinning of trees) to favor oak/hickory regeneration and slow down or reverse this trend towards shade-tolerant tree species. While these forest management tools are being used to alter forest structure and tree species composition, as avian ecologists our interest has been to better understand how these widely used management tools (particularly prescribed fire) affect the relative abundances and nesting success of birds breeding in forests where management is occurring.

Cowbirds are a brood parasite and always lay their eggs in the nests of other species (i.e., hosts). Raising cowbirds is costly to hosts who subsequently raise fewer or none of their own chicks if parasitized. Illinois forests contain many species of songbirds that serve as hosts for cowbirds. Many hosts nest on or near the ground (e.g., Kentucky and worm-eating warblers and ovenbirds) or in shrubs (e.g., hooded warblers, indigo buntings, white-eyed vireos and northern cardinals) in the forest understory. Because prescribed fires burn through the leaf litter and understory vegetation, many of the species mentioned above are less abundant in forests burned during the winter/spring immediately preceding the summer breeding season. We thought that this could potentially affect the cowbird parasitism rates for the remaining hosts nesting in the sub-canopy/canopy of the forest whose abundances are not affected by prescribed fire.

During the summer of 2022, we located and documented parasitism status for 118 Acadian flycatcher nests. Parasitism rates were higher at Hidden Springs State Forest than at Trail of Tears State Forest (Table 1) and rates at Trail of Tears were unusually low compared to what they were historically. The higher overall rates of parasitism at Hidden Springs were likely a function of that forest being a much more fragmented and relatively narrow stretch of forest embedded in a mostly agricultural landscape relative to Trail of Tears. We speculate that the surprisingly low rates of parasitism at Trail of Tears, compared to historical values, may stem from the closure of the on-site tree farm and the conversion of a large cattle pasture west of the forest—both were known to serve as foraging habitat for large numbers of local cowbirds. Very low rates of parasitism at Trail of Tears suggest the site may contribute to maintaining or enhancing populations of songbirds in the region.

At each site, the proportions of nests parasitized across the three time-since-fire categories were quite similar with a tendency for only slightly higher values in the freshly burned units (Table 1). This preliminary data suggests that time-since-fire does not have a substantial effect on cowbird parasitism of Acadian flycatchers, which is good news for those concerned that the temporary disturbance in the forest created by prescribed fire might increase rates of cowbird parasitism. To further validate this preliminary result, we plan to collect another year of nesting data at these two sites and add Stephen A. Forbes State Recreation Area and Ramsey Lake State Recreation Area. This additional nesting data, in conjunction with bird survey data documenting host and female cowbird abundances in each management category (i.e., freshy, recently, or not burned), will further elucidate how birds respond to the habitat mosaic created in forests being managed with prescribed fire. Stay tuned!

This project is funded with federal pass-through funds from the Illinois Department of Natural Resources (specifically money from the Federal Aid in Wildlife Restoration Act) associated with the Forest and Woodlands Campaign of the Illinois Wildlife Action Plan.

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Dr. Jeff Hoover is an Avian Ecologist with the Illinois Natural History Survey, Prairie Research Institute, University of Illinois. Jeff has been studying forest songbirds since 1990 in upland and bottomland forests, and in Appalachian and Midwest forests.

Alex Lowe-Massi is a graduate student in The Department of Natural Resources and Environmental Sciences at the University of Illinois. Alex is an accomplished birder and a budding ornithologist.