Bald Eagle State Park, Central Pennsylvania

Helping Warblers and Woodcock (and Other Wildlife, Too)

At this park in Centre County, Pennsylvania, 5,900 acres of land surround a large flood-control lake. When the U.S. Army Corps of Engineers bought the acreage in 1965, most of it was farmland. Since then, the fields have grown up in brush and forest. Today Bald Eagle is a popular destination for birders, hunters, and other outdoor enthusiasts. To keep the park wildlife-friendly, the Pennsylvania Department of Conservation and Natural Resources – along with several other agencies and organizations – are creating young forest habitat while promoting scientific research.

Lush regrowing habitat at Bald Eagle. Many birders visit this popular state park to observe young-forest species, including American woodcock and golden-winged warblers./C. Fergus

Many of the old fields are choked with autumn olive, multiflora rose, and Asian honeysuckle. Those non-native invasive shrubs supply some food and cover for wildlife, but they don’t do as good a job of it as native shrubs. There’s been a big push at Bald Eagle to combat the exotic shrubs, with conservationists cutting them down and using herbicides to keep them from growing back, then planting native shrubs along with fast-growing aspen trees. Since 2002, they’ve created and restored more than 157 acres of young-forest habitat.

Jeff Larkin is a conservation biologist at Indiana University of Pennsylvania. He supervised a four-year study of golden-winged warblers at Bald Eagle from 2008 through 2011, capturing and banding the birds and mapping their territories. “We looked at areas the warblers used for breeding, as well as areas they didn’t use,” Larkin says. “We identified the kinds of shrubs that grew in the habitats that the birds favored, and recorded their height and density along with other vegetation characteristics.”

Where the Warblers Are

Male golden-winged warbler, captured in a mist net, ready to be banded./J. Larkin

Golden-winged warblers nest on or near the ground, often in a tangle of blackberry or in thick goldenrod at the base of a shrub. Says Larkin, “We tracked nesting success – how many of the warbler pairs successfully fledged young, and how often their nests failed.”

Altogether Larkin and his research team monitored around 30 golden-winged warbler pairs in the park each year. “Frankly, nesting success wasn’t great, even in good habitat: around 30 percent. Predation was a significant factor. We saw it again and again. Nestlings cleaned out and the nest left intact? That’s a snake or a chipmunk. The nest pushed down and wrecked? Probably a larger mammal, a raccoon or opossum or skunk.”

Larkin’s findings are part of a seven-state effort to learn why the population of the golden-winged warbler (or GWW, in biologist-speak) has been plummeting. The data help inform the Golden-Winged Warbler Conservation Plan, a document spelling out the bird’s status, habitat use, and how to reverse its decline.

This handsome insect-eating warbler was once much more common than it is today. Breeding Bird Survey results suggest a yearly population drop-off of 9 percent in the northcentral states and a frightening 20 percent in the Northeast. Scientists think this dwindling comes not just from predators but also (and probably more important) from a loss of habitat: old fields swallowed up by development, a loss of key wetlands areas, and shrubland and thickets maturing to become forest, where golden-wings don’t live.

Not Enough New Habitat On the Way

“Today, not enough new habitat is being made,” Larkin says. “Very few farm fields are being abandoned and allowed to turn into brush. Not enough timber harvesting is taking place to create adequate acres of new, regrowing young forest. With current fire suppression, very few big fires come along to burn down trees and make large expanses of new young forest.” Even flood-control dams, like the one at Bald Eagle State Park, prevent big floods and ice scouring that once lay waste to significant acreages of forest, leading to their regrowing as dense habitat.

Wildlife-friendly young forest growing at Bald Eagle State Park./C. Fergus

At Bald Eagle, Larkin teamed up with biologists making habitat for American woodcock, and together they came up with a way of creating young forest that helps both woodcock and golden-wings. Instead of simply cutting down all the old or straggling shrubs or middle-aged trees in a habitat strip or block (so that the shrubs and trees can regrow more densely and provide better cover for wildlife), it works just as well to leave a few scattered shrubs and trees. These don’t impede woodcock nesting and brood-rearing, and they provide perches where male golden-wings can sing and advertise their territories. “Routinely we would find woodcock nests and flush woodcock hens and their young when we were checking on golden-wing nests,” Larkin notes.

“We learned a lot during the study. For instance, we found that we really need to knock back the invasive shrubs with herbicides. Cutting alone won’t turn an area thick with invasives into lasting, good-quality GWW habitat: Without the herbicides, the invasive shrubs come back so thick and so fast that an area will grow out of being usable to golden-wings in just a few years.” In dense shrub thickets, “The goal is to create and maintain shrub cover intermixed with openings that support the goldenrod, grasses, and blackberry that golden-wings need for nesting.”

Birds Coming Back

Not just golden-wings and woodcock are thriving in the new young forest and shrubland created at Bald Eagle. “Where we were successful at breaking up the monoculture of invasive shrubs,” Larkin said, “things really started happening. We saw a strong response from birds like yellow-breasted chats, brown thrashers, prairie warblers, indigo buntings, and willow flycatchers. I got a good feel for the increase in avian diversity just by walking through the areas where we did the cutting and hearing all the bird songs in spring.”

In a related project, Larkin and his colleagues also made three 25-acre patches of young forest on Sproul State Forest, 20 miles north of the park and a thousand feet higher in elevation. There, the soil is drier and rockier than at Bald Eagle, with more forest and almost none of the invasive shrubs that plague the old farmland in the park. After the patches on the state forest were cleared of low-quality trees, the sites began growing back in thick young trees as well as dense blueberry and blackberry shrubs. “Past research has shown that golden-wings will start breeding in such cleared forest stands after 4 to 5 years and will keep using the stands for another 12 to 15 years,” Larkin says.

Clearing away low-value timber on Sproul State Forest. Native shrubs and young trees sprang up on this site, providing great habitat for golden-winged warblers, whip-poor-wills, Appalachian cottontails, and many other animals./J. Larkin

The Pennsylvania Bureau of Forestry, a partner in the Sproul State Forest study, is working with Larkin to explore ways of making better wildlife habitat on sites that the agency can’t economically manage for high-quality timber – as well as figuring out how best to harvest trees in better-quality woodlands to create the young forest needed by golden-winged warblers, Appalachian cottontail rabbits, whip-poor-wills, and a host of other species.

What are the take-home lessons Larkin learned at Bald Eagle State Park and in Sproul State Forest? “Because of its history and the existing shrubland habitat, it’s important to manage the park as good-quality shrubland,” he says. “But we need to keep in mind that that kind of management is labor-intensive and expensive.

Best Way to Make Habitat

“I think it’s significantly easier to create new habitat for golden-wings through timber harvesting. If you log an area of forest, it will likely be a number of years before the populations of snakes and other predators spike high enough to cut into the warblers’ nesting success. I think we’ll have a higher chance of increasing GWW numbers through that kind of management.

“We also need to protect and enhance the natural breeding habitat in swamps – places where golden-wings have historically done well. In wetlands, golden-wings nest on sedge hummocks surrounded by water, which shields them from mammalian predators – you won’t find chipmunks or opossums or rat snakes getting to the nests as often.

“It’s important to look at things on a landscape scale, to make sure different ages of forest are scattered throughout a state or region. We need to manage properties that are large enough so that we don’t have to keep going back to the same place, like we’re doing at Bald Eagle. By moving young forest around on the landscape, we can provide ample habitat while staying ahead of the predators to a certain extent.”

Larkin has expanded his work on golden-wings from pure research to applied science and outreach. As of 2012, he’s overseeing four full-time conservation foresters who are developing management plans for both public and private lands in a Golden-Winged Warbler Focal Conservation Area that takes in parts of 30 Pennsylvania counties. In a little over a year, the foresters have prepared young-forest habitat projects on more than 6,000 acres, with some of the projects already underway or completed. Some projects will yield revenue from the sale of forest products. Others are noncommercial jobs funded by the Pennsylvania Game Commission, the U.S. Department of Agriculture, the Natural Resources Conservation Service, and the National Fish and Wildlife Foundation.

Larkin is optimistic about how these habitat-creation projects will help the beleaguered golden-winged warbler. “If we’re going to reverse the decline of golden-wings, woodcock, and other young-forest wildlife,” he says, “it’s going to start here in Pennsylvania.”

Funding and Partners

Pennsylvania Department of Conservation and Natural Resources, Pennsylvania Game Commission, U.S. Army Corps of Engineers, U.S. Fish and Wildlife Service, U.S.D.A Natural Resources Conservation Service, Ruffed Grouse Society, California University of Pennsylvania, Indiana University of Pennsylvania, Wildlife for Everyone Endowment Foundation, Woodcock Limited of Pennsylvania, U.S. Geological Survey, National Fish and Wildlife Foundation, Northeast Association of Fish and Wildlife Agencies, Wildlife Management Institute.

How to Visit

Bald Eagle State Park lies along PA Route 150 between Milesburg and Lock Haven. It is open, free, to visitors year-round. For details and a downloadable map, see http://www.dcnr.state.pa.us/stateparks/parks/baldeagle.aspx.

The Pennsylvania field office of the U.S. Fish and Wildlife Service, in nearby State College, PA, coordinates the Native Plant and Early Successional Stage Habitat Restoration Project at Bald Eagle State Park. For more information, contact Adam Smith at 814-234-4090 x 235 or adam_smith@fws.gov.

The website of the Golden-Winged Warbler Working Group offers up-to-date information on this species.