Managing Forests for Birds in Indiana
By Kenneth F. Kellner, Patrick J. Ruhl, John B. Dunning Jr., Robert K. Swihart
Indiana hosts a diverse community of bird species. As both predators and prey, birds are important elements of nature’s food web. Many birds also play important roles as pollinators and seed dispersers. In addition to their ecological value, their beautiful colors and songs are enjoyed by birdwatchers throughout the state.
The diversity of bird species native to Indiana is matched by their diverse habitat requirements. For example, some species, like the Ovenbird and Red-eyed Vireo, prefer forested habitat with large, older trees and a closed canopy. Other species, like the Indigo Bunting and Ruffed Grouse, prefer forested habitat with dense, shrubby vegetation (these are often called “shrubland” species). More than half of birds found in the Midwest actually use a mixture of habitat types throughout their lifetimes - for example, Worm-eating Warblers are frequently found in mature forest but also make use of young forest habitat to find food and hide from predators.
Over the last several decades, scientists have observed declines in numerous bird species across the eastern United States. Many of these declines have been attributed to habitat loss as humans continue to alter the natural landscape, most recently with conversion of habitat for development. Many people focus their attention on the loss of mature, closed-canopy forest habitat. However, the loss of young forest habitat is of equal or greater concern. In fact, closed-canopy forest has increased by approximately 15% in Indiana since 1950, while the amount of young forest habitat continues to decline throughout the Midwest.
Loss of young forest habitat has resulted from changing patterns of forest disturbance. Prior to European settlement of Indiana, patches of dense, young vegetation were constantly created by natural disturbance events like wildfire, tornadoes, and insect infestations, as well as by fires set intentionally by Native Americans. Indiana forests are highly resilient to such disturbances. Thus, over time, these patches naturally regenerated back to closed-canopy forest, and birds that specialized on young forests moved on to the next disturbance opening. This cycle of disturbance, regeneration, and maturation ensured a diversity of habitat types across the landscape that were capable of maintaining a diversity of bird species.
As human populations have increased and technology has progressed, human influence has grown across the Indiana landscape. Forests that once spanned the entire state have now been broken apart into smaller pieces. Furthermore, natural disturbances (like fire) are perceived as dangerous or unappealing to humans, and thus have been suppressed. As a result, the natural cycle of disturbance and regeneration has been interrupted the amount of young forest habitat has declined, and so have the populations of birds that depend on this habitat. Indeed, nearly three-quarters of young forest specialist bird species are currently declining in abundance based on North American Breeding Bird Survey data.
Conservation of the bird community of Indiana requires wise stewardship of our remaining Indiana forests including preservation of mature, closed-canopy forest and creation new patches of young forest habitat. Luckily, there is a flexible set of tools available for forest management, including timber harvesting and prescribed burning. But a key question remains: among the many options, which forest management approaches are likely to be most beneficial for birds and other wildlife?
The Hardwood Ecosystem Experiment
Careful research is necessary to determine which management approaches are most beneficial to the breeding bird community. The Hardwood Ecosystem Experiment (HEE) seeks to provide science that examines the effects of different types of forest management in Morgan-Monroe and Yellowwood State Forests.
Three forest management treatments occur at the HEE: No harvest (as a control), even-aged management, and uneven-aged management. The goal of the even-aged treatment is to create a future forest stand where all trees are of a similar age, using a mixture of 10-acre clearcut and 10-acre shelterwood timber harvests. In the uneven-aged treatment, smaller 1-5 acre patch cuts and single-tree selection are implemented with the goal of a future forest stand of mixed age.
In each treatment, we monitored bird abundance over a 10-year period both before and after the timber harvests occurred, which allowed us to account for any pre-existing differences in the bird community between sites.
Prior to harvesting, our study sites were intact, closed-canopy forest. Unsurprisingly, the most common bird species present were those associated with this type of habitat including the Ovenbird, Red-eyed Vireo, and Worm-eating Warbler. In contrast, “shrubland” species associated with young forest habitat (like the Indigo Bunting, Carolina Wren, Yellow-breasted Chat, and Eastern Towhee) were relatively uncommon.
Our results show that the young forest created following harvest provided habitat for shrubland species, increasing their abundance. Among the species that responded positively to timber harvesting were several species of conservation concern in Indiana, including the Cerulean Warbler, Hooded Warbler, and Black-and-white Warbler (Figure 1). Some species that preferred intact, closed-canopy forest habitat (like the Worm-eating Warbler) showed modest declines in abundance following timber harvest since they no longer had quite as much suitable habitat available. However, plenty of intact forest habitat remained and these species likewise remained abundant; for example, the Worm-eating Warbler remains several times more abundant than the young forest species in both management treatments (Figure 1).
When looking at the entire bird community, we found that both even- and uneven-aged management treatments increased the total number of bird species (that is, the “richness”) at those sites. Figure 2 illustrates that prior to harvesting, richness was similar among the three treatments. After the harvests were applied, richness increased quickly at the even- and uneven-aged management sites relative to the no-harvest control sites. By 2014, richness was nearly 20% higher in the even- and uneven-aged treatments relative to the no-harvest control.
In both the even- and uneven-aged harvest treatments, timber harvesting increased the diversity of habitat types available to the bird community by creating patches of young, dense forest. These patches provide valuable habitat for shrubland birds, and are used for food and cover by some mature forest species as well. Overall, timber harvesting resulted in increased bird species richness corresponding to increased habitat diversity.
Forest landowners and managers seeking to promote a diverse community of forest bird species have multiple options. The largest, most intensive timber harvests in our study (10-acre clearcuts) resulted in the highest richness and largest increases in shrubland birds. In cases where openings that large may not be feasible, smaller patch cut openings (1-5 acres) were also used by many shrubland species. Of course landowners and managers should also strive to preserve patches of mature, closed-canopy forest habitat to maximize the diversity of habitats available to the forest bird community. This is especially true in fragmented landscapes that already contain abundant young forest habitat but little closed-canopy forest.
For more information about this study, see our recent paper “Multi-scale responses of breeding birds to experimental forest management in Indiana, USA” (Forest Ecology and Management 382: 64-75).
Ken Kellner is a post-doctoral research assistant at Purdue University. He is using the infrastructure of sites at the Hardwood Ecosystem Experiment to develop and parameterize multi-scale models of abundance and habitat use by breeding birds. Patrick Ruhl is a PhD student and is currently studying how song birds respond to timber harvesting. Barny Dunning is a professor of wildlife ecology at Purdue University. Rob Swihart is a professor of wildlife science at Purdue University.