Approximately ten acres of early successional habitat in Leominster, Massachusetts. Species using this habitat included Prairie Warblers and Field Sparrows. Photo was taken two-three years post-treatment. Photo by Jeff Ritterson.
Massachusetts is the eighth most forested state in the country. It is also the third most densely populated, with many people living among the state's approximately three million acres of forest. These forests provide residents with a range of ecosystem goods and services. For example, forests influence the quality and quantity of drinking water, provide flood and erosion control, sequester carbon which has climate change implications, and yield forest products such as timber and maple syrup. Of course, forests also support a diversity of wildlife that deliver additional goods and services and provide recreational opportunities, such as birdwatching.
The soils and climate of Massachusetts support vigorous forest growth. Following a disturbance, a forest will naturally regenerate, a process that may begin with herbaceous plants and perennial wildflowers and continue with the establishment of woody shrubs and saplings. Eventually a tall canopy of trees will form and be easily recognized as a forest. This successional process influences habitat conditions and what species of birds inhabit a forest.
Current habitat conditions in Massachusetts can be explained by a complicated land use history. Large-scale forest clearing began with European colonization in the 1600s and 1700s and, by the end of the 19th century, some 70% of forests had been cleared for pasture, farmland, and orchards. Fuel wood and timber were extracted from most other forests, and little virgin old growth remained. The rise of urban-industrial centers and better farming elsewhere led to the abandonment of farmlands, and the successional process began. However, new-found markets led to a second clearing of forests in the early 1900s. These markets declined between 1920 and 1950, and the forests were left to regrow. Now, most forests in Massachusetts are considered to be middle-aged, at approximately 80 to 100 years old. There is little young forest and less old growth (de la Crétaz et al. 2010).
Although it is difficult to neatly categorize each species, for the sake of simplicity forest birds can be placed into two groups: those that breed within early successional habitats (e.g., shrubland and young forest) and those that breed within older, closed-canopy forests.
Early successional birds are categorically in decline, as evidenced by long-term bird monitoring programs such as the North American Breeding Bird Survey and the Massachusetts Breeding Bird Atlas. Of the 41 species identified by Schlossberg and King (2007) as early successional breeders in New England, 33 (80%) require conservation attention, with declines in abundance concentrated in southern New England. Although other factors can contribute to population declines, these losses can largely be attributed to the loss of breeding habitat.
Examples of early successional birds in decline include Ruffed Grouse, American Woodcock, Whip-poor-will, Brown Thrasher, Blue-winged Warbler, Chestnut-sided Warbler, Prairie Warbler, Mourning Warbler, Canada Warbler, Eastern Towhee, Field Sparrow, and White-throated Sparrow.
Other species considered to be early successional breeders are doing well, including Cedar Waxwing, Carolina Wren, Gray Catbird, and Northern Cardinal. These species may be thriving due to their ability to successfully nest within suburban yards. Also, Carolina Wren and Northern Cardinal have spread their ranges northward in recent decades, greatly contributing to their increased abundance in the state.
Young forests have traditionally composed the bulk of early successional habitat in Massachusetts. However, young forest is an ephemeral resource—suitable habitat is lost after 15–20 years of regeneration. Most forests have matured past the point of suitability for early successional birds, contributing to the loss of over 90% of this habitat in Massachusetts since 1950—a trend seen throughout southern New England.
Young forests are dependent on medium to large scale disturbances to reset the successional clock. Historic sources of disturbance include flooding from beaver activity, fire (particularly in the pitch pine-oak forests of the southeast), insect outbreaks, Native American activities, and major wind and ice storms. Of course, we now suppress beavers and fire, and Native Americans are no longer heavily influencing the landscape. Furthermore, our middle-aged forests are less susceptible to storm damage.
Estimated acreage of forest age classes in Massachusetts (USDA Forest Service FIA 2018).
Current sources of early successional habitat include wildlife management areas and utility rights-of-way that are specifically managed for this habitat. Forestry operations can create areas of young forest, yet a negative public perception of forestry and a struggling forest economy have reduced these practices. In some circles, "clear-cut" is considered a dirty word. In reality, it is simply a silvicultural term. Appropriately sized and well planned clear-cuts and related practices have proven to be effective in creating habitat for early successional birds.
Birds that breed within older, closed-canopy forest are collectively faring better than their early successional counterparts. These species include Sharp-shinned Hawk, Red-bellied Woodpecker, Blue-gray Gnatcatcher, Pine Warbler, Black-throated Green Warbler, Blackburnian Warbler, American Redstart, Ovenbird, Yellow-throated Vireo, and Winter Wren.
However, some species are experiencing long-term declines such as Wood Thrush, Scarlet Tanager, Veery, Northern Flicker, and Rose-breasted Grosbeak. Declines can be caused by factors on the breeding grounds. For example, species like Wood Thrush are area sensitive, and the fragmentation of forests into smaller areas maybe be causing low breeding success. Also, microhabitat features such as understory vegetation and cavity trees for nesting Veeries and Northern Flickers, respectively, may be deficient. Alternatively, factors during the migratory or wintering periods (e.g., building strikes and tropical deforestation) also may contribute to declines for migratory species. As a result, many research efforts are now focused on identifying conservation issues and actions throughout a species entire year-round range.
Even the most long-term monitoring programs only capture relatively recent population trends, and it is possible that currently stable species were more abundant several centuries ago. Regardless of their past numbers, all species could stand to have their populations bolstered to help overcome current and future conservation challenges, and enhancement of their breeding habitats can contribute to this goal.
High quality habitat is often described as structurally complex. A layer of leaf litter and other organic material on the ground provides nesting habitat for species like Ovenbird. Patches of an understory, up to five feet tall, are important for species like Black-throated Blue Warbler to nest. Areas of a developed midstory, five to 30 feet tall, are important for birds like Wood Thrush. Finally, a relatively tall canopy is where species like Scarlet Tanager nest. Other features of high quality habitat include snags (dead standing trees), which are decomposing and providing arthropod prey, cavity trees for certain species to nest in, logs and other woody debris on the ground, and gaps in the canopy.
These features of structural complexity are typical of old-growth forests and are generally reduced or absent in the middle-aged forests common in Massachusetts. Unfortunately, old-growth forest conditions will not naturally develop until a forest is at least 150 to 200 years old. However, the conservation value of emulating old-growth conditions with forestry practices has been demonstrated and can be used to enhance forest bird habitat (Rankin and Perlut 2015).
Habitat management through sustainable forestry is widely promoted by various state and federal agencies, academic institutions, and bird conservation organizations, but accomplishing on-the-ground habitat management has been easier said than done, in part due patterns of land ownership.
The federal government owns an insignificant amount of forest in Massachusetts. The state government, however, owns about 20% of the forests in Massachusetts, which fulfill various goals, including wildlife habitat and large areas of reserves where—appropriately—no management will ever occur. Meanwhile, about 65% of Massachusetts' forests are in private family ownership. To scale forest bird conservation efforts up to a more meaningful level, some management will have to occur on private lands, though implementing this approach has proved challenging.
In recent years, a conservation strategy has emerged to conduct habitat management on private lands. The program, titled "Foresters for the Birds," emerged in Vermont during discussions between Audubon Vermont and the Vermont Department of Forests, Parks and Recreation. Several other states in the region have adopted the program, including Maine, Massachusetts, New York, and Connecticut.
In Massachusetts, the program is a partnership among the Department of Conservation and Recreation, the Massachusetts Woodlands Institute, and Mass Audubon. Consulting foresters, who work directly with landowners, are trained to assess the current habitat conditions in a forest and then write a forest management plan featuring habitat management recommendations. This plan could include creating young forest habitat or enhancing the structural complexity of middle-aged forests. The landowner may also have other goals, such as management for timber or recreational opportunities, which need not preclude management for bird habitat.
By merging the forest management expertise of professional foresters and the ornithological experience of Audubon biologists, the program provides the technical assistance many landowners need to manage for birds, and that service is in demand. According to National Woodland Owner Survey data (Butler et al. 2016), almost 70% of respondents in Massachusetts indicated a desire to protect or improve wildlife habitat as a reason for owning land. Meanwhile, only about 26% of landowners have a forest management plan.
The conservation issues affecting forest birds are not only rooted in past land use history and human activity, but also in future environmental considerations. The forests of Massachusetts face many challenges that can alter ecosystem functions and spur the degradation or loss of habitat.
For example, parts of the state are facing high rates of development and forest fragmentation. Invasive plants are displacing native species and altering ecosystem processes. High densities of deer are overbrowsing forests—of particular detriment to understory nesting birds. Insects and disease are affecting various tree species, and climate change is affecting our forests through complex pathways.
These stressors do not act alone, but can work interactively to exacerbate each other's effects. For example, white-tailed deer preferentially browse on native plants, giving invasive species a greater competitive advantage. Climate change is expected to increase the prevalence of drought conditions, which will stress some tree species and make them more susceptible to insect pests.
While these issues are daunting, the conservation community recognizes several strategies to overcome forest stressors, mostly stemming from everyday conservation actions.
First and foremost, keep forests as forests. Simply put, no forests means no ecosystem goods and services. Local land trusts and conservation estate planning are important resources to combat forest loss.
Second, reduce as many stressors as possible. This work could include managing invasive plants or controlling deer populations. Robust populations of, for example, trees and birds provide the individual and genetic variability needed to overcome and respond to changes in environmental conditions and habitat. By extension, other causes of mortality for birds can be reduced. For example, domestic cats are estimated to kill 1.4 to 3.7 billion birds (and >6 billion small mammals) in the United States every year (Loss et al. 2013). Many prey items are not brought home, so some cats may appear to not be hunters (Loyd et al. 2013). A simple solution is to keep cats indoors.
A final strategy is to maintain or increase the resiliency or responsiveness of our forests (Millar et al. 2007). A resilient ecosystem is able to withstand a stressor and return back to normal in a relatively short amount of time. A responsive ecosystem may undergo a certain amount of change caused by prevailing conditions, yet retain its basic functionality. For instance, a resilient forest will be multi-aged with a diversity of tree species and tree ages. This concept is akin to putting eggs in many baskets to protect against changing conditions. These changes are somewhat predictable, so retaining or encouraging the growth of tree species predicted to do well in, for example, future climate conditions can increase the responsiveness of a forest.
Resilient and responsive forests are more easily able to withstand stressors and seamlessly provide habitat for birds and other wildlife. Luckily, many of the same recommendations to address habitat issues arising from our land use history are compatible with resiliency planning.
While the forest birds of Massachusetts are certainly not without conservation concern, there is an active and engaged conservation community in the state working to protect forests and the wildlife they support. The success of these efforts hinges once again on land use choices. Harvard Forest and their collaborators describe their vision of a sustainable land use future in their Wildlands and Woodlands report (Foster et al., 2017). They suggest that 7% of New England be protected as forest reserves, termed wildlands, where natural processes will shape the landscape over time, and 63% be used as woodlands, which are managed to provide many services, including bird habitat. That's 70% of New England protected as forest! The remaining land area is devoted to farmland, cities, and communities.
Accomplishing this vision of a sustainable future requires the engagement of the public, and includes changes in behavior, such as reduced consumption rates and dietary shifts. There is plenty of work to do, but together we can achieve sustainability, including a thriving assortment of forest birds to enjoy and celebrate.
Jeff Ritterson is a Field Ornithologist with Mass Audubon, where he focuses on working landscapes, including the coordination of the Massachusetts Foresters for the Bird program.
- Butler, B. J., J. H. Hewes, B. J. Dickinson, K. Andrejczyk, S. M. Butler, M. Markowski-Lindsay. 2016. USDA Forest Service National Woodland Owner Survey: national, regional, and state statistics for family forest and woodland ownerships with 10+ acres, 2011-2013. Research Bulletin NRS-99. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 39p. Available online at: https://www.fs.fed.us/nrs/pubs/rb/rb_nrs99.pdf
- de la Crétaz, A. L., L. S. Fletcher, P. E. Gregory, W. R. VanDoren, and P. K. Barten. 2010. An assessment of the forest resources of Massachusetts. Prepared for the USDA Forest Service. Available online at: http://www.mass.gov/eea/docs/dcr/stewardship/forestry/assessment-of-forest-resources.pdf
- Foster, D.R., K.F. Lambert, D. Kittredge, B. Donahue, C. Hart, W. Labich, S. Meyer, J. Thompson, M. Buchanan, J. Levitt, R. Perschel, K. Ross, G. Elkins, C. Daigle, B. Hall, E. Faison, A. D'Amato, R. Forman, P. Del Tredici, L. Irland, B. Colburn, D. Orwig, J. Aber, A. Berger, C. Driscoll, W. Keeton, R. Lilieholm, N. Pederson, A. Ellison, M. Hunter, and T. Fahey. 2017. Wildlands and Woodlands: Farmlands and Communities, Broadening the Vision for the New England. Harvard Forest, dist. by Harvard University Press, Cambridge, Massachusetts. 36pp. Available online at: http://wildlandsandwoodlands.org/sites/default/files/W%26W%20report%202017.pdf
- Loss, S. R., T. Will, and P. P. Marra. 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications 4: 1-7. doi: 10.1038/ncomms2380
- Loyd, K. A. T., S. M. Hernandez, J. P. Carroll, K. J. Abernathy, and G. J. Marshall. 2013. Quantifying free-roaming domestic cat predation using animal-borne video cameras. Biological Conservation 160: 183–189.
- Millar, C. I., N. L. Stephenson, and S. L. Stephens. 2007. Climate change and forests of the future: managing in the face of uncertainty. Ecological Applications 17 (8): 2145–2151.
- Rankin, D. T., and N. G. Perlut. 2015. The effects of Forest Stand Improvement Practices on occupancy and abundance of breeding songbirds. Forest Ecology and Management 335: 99–107.
- Schlossberg, S., and D. I. King. 2007. Ecology and management of scrub-shrub birds in New England: a comprehensive review. Submitted to the USDA Natural Resources Conservation Service, Resource Inventory and Assessment Division. Available online at: https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_013252.pdf