By Rex Graham
Beautiful Blue-headed Vireos, American Redstarts, Scarlet Tanagers and dozens of other hard-to-find songbirds that nest in mature forests relocate to an odd habitat after the fledging stage. Adults and young birds abandon the stately, deep woods for less picturesque areas of brush, bramble and short trees in areas recovering from clear-cutting by timber companies.
In these so-called early-successional habitats, post-breeding forest birds fatten up in preparation for fall migration amid the thorns, berries, wild fruit trees and plant-eating invertebrates. When parenting is done for the season, forest birds find the “cuts” irresistible, according to a new study published in summer 2013 in The Auk.
Forest birds relocate
Scott H. Stoleson, author of the study and an ornithologist with the U.S. Department of Agriculture’s Forest Service, said his four-year mist-netting study of the birds of northwest Pennsylvania documented a profound post-breeding preference by 31 of 33 species of songbirds for recovering clear-cut areas.
The discovery of a preference for the cuts by post-breeding forest birds is not new. The phenomenon also has been documented in southeast Ohio, the Missouri Ozarks, White Mountain National Forest of New Hampshire, and bottomland forests of coastal South Carolina.
The first study documenting post-breeding forest birds’ preference for cuts to be published in The Auk, a distinguished quarterly journal published by the American Ornithologists’ Union, is an important signal to ornithologists worldwide.
The publication, which is available in 70 languages, carries a clear scientific and political imperative to re-think the possible benefits of regenerating cuts.
‘Interesting conservation dilemma’
Post-breeding songbirds that prefer cuts over protected national forests have already created an “interesting conservation dilemma,” according to Stoleson and other ornithologists who are piecing together the peripatetic lives of forest birds.
The findings in The Auk study are as surprising as they are well proven. It makes environmentalists cringe awkwardly and timber company owners smile. Why? Birds that require extensive tracts of mature forests for breeding are in decline, and yet clear-cutting appears to be a mechanism to provide critical post-breeding habitat for many of those same birds.
For their part, anti-clear-cutting groups are armed with hundreds of studies that document the ecological devastation caused by many clear-cutting practices. But not all cuts are the same. Also, natural versions of cuts are caused by forest fires and other natural phenomena. Plants and animals adapt.
Still, not all ornithologists agree on the importance of cuts to post-breeding forest birds. Solid scientific findings like those reported by Stoleson should help environmentalists and timber companies compromise on policies that benefit the environment, birds and other animals, and society.
Science of ‘cuts’
The science of cuts is evolving. Even Stoleson says much more research is needed. Complicating the situation is that forest understory plant communities, the bird species that inhabit them, and the political clout and persuasion of local Congressional representatives vary widely from place to place.
“In New Brunswick, Canada, where mature forest has been declining for many years and cuts are plentiful, mature-forest-associated species continue to decline rapidly,” said Matthew Betts, an associate professor in the Department of Forest Ecosystems and Society at Oregon State University.
He said that while Stoleson’s study “seems quite well done,” birds that spend some time in the canopy of mature forests would be more likely to be undercounted because they would fly over mist nets stretched within the understory.
“Even if cuts are important in the post-fledging stage,” Betts said, “the trick will be to find the balance between early-stage and mature-forest breeding habitat.”
Birders interested in adding forest birds to their life lists may try two approaches. Visit state or national forests with up to hundreds of thousands of acres of contiguous mature trees during the spring breeding season. “Alternatively, wait until July or August and find a scrubby regenerating cut within such a forest,” Stoleson said.
However, the birds in cuts tend to vocalize much less than they do in their nesting territories: birders won’t be able to rely on vocalizations to locate a Black-throated Green Warbler or a Veery. Also, the dense vegetation of cuts allows birds to more easily hide from predators and birders.
Two remaining questions
Do birds that nest in mature forests derive a “fitness benefit” by switching to cuts after the breeding season is over? The major contribution of Stoleson’s study was the clear demonstration of differences in the condition, or overall fitness, of understory birds: the birds capured in the cuts were in much better condition than those netted in mature-forest locations.
The second questions centers on the amount of time that post-breeding forest birds spend in cuts. Stoleson pointed out that it’s possible, however unlikely, that forest birds simply use cuts as avenues to quickly fly from one area of mature forest to another. Radio-tracking individual birds should determine if post-breeding birds travel that way.
There were three species that showed no preference for either mature forests or cuts. Swainson’s Thrush and Hermit Thrush were netted in nearly equal numbers in both locations. Understory-foraging Hooded Warblers were captured more frequently in cuts, but the number was not significantly different in either location.
Since the Swainson’s Thrush, Hermit Thrush, Ovenbirds and Worm-eating Warblers prefer dense vegetation in both the cuts and mature forests, where would they go if the understory foliage is over-browsed by deer, which is a common problem in mature forests? My suspicion is that they will prefer the cuts.
Pennsylvania forest survey
Comprehensive continent-wide assessments of the post-breeding preferences of forest birds will undoubtedly rely on long-term mist-netting surveys similar to that used by Stoleson. He captured 3,845 individual birds of 46 species. He set up his nets during the post-breeding season from 2005 to 2008.
He used four netting locations in the Allegheny National Forest, and four netting locations on private industrial timberland holdings four to eight years post-clear-cut. All eight netting sites were in northwest Pennsylvania.
Of the 33 species of birds with at least 10 individuals netted, 31 species were found in significantly higher numbers in the cuts than in mature forests. For example, 26 Blue-headed Vireos were netted in cuts, but only four were netted amid the mature trees of the national forest. Stoleson netted 68 American Redstarts in the timber company cuts, and only one in the Allegheny National Forest.
A clear-cut banquet
It’s as if Stoleson’s study revealed a banquet for birds in cuts. One of many clues was the frequent berry “staining” of the beaks and throat feathers of thrushes, waxwings and catbirds captured in the nets positioned in cuts. On the contrary, he said he found very little berry staining on the birds netted in mature forest locations.
Indeed, the cuts produce large crops of fruit in late summer, including elderberries, raspberries, blackberries, and dewberries, as well as Pin Cherry and many types of invertebrate-harboring plants, all of which are almost absent from mature forests.
Despite the low stature of plants in the cuts, their volume of leaves was similar to that of mature forests. Combined with all the berries and a highly concentrated supply of plant-eating invertebrates, the cuts are a culinary delight for birds.
What about western birds?
No two forests are the same. The forests of Pennsylvania, dominated by Black Cherry, Red Maple, Eastern Hemlock, Sweet and Yellow Birch, are not like the forests of the Pacific Northwest, which have Douglas Fir, Larch, Mountain Hemlock, Bigleaf Maple, Lodgepole Pine and other trees not found in the eastern U.S. And just as important, the understory plants in the forests of Oregon and Pennsylvania are very different.
“Our experience in Oregon has been that we do not see the same use of clear-cuts by mature forest species as has been observed in the East,” Betts said. However, there are few studies of the post-breeding forest birds in the West, and Betts said he has not yet submitted his observations to a peer-reviewed journal.
“Betts makes some good points: clearly in New Brunswick, the high proportion of land in regenerating cuts limits populations of birds that breed in mature-forests,” Stoleson said. In contrast, in northwest Pennsylvania it’s estimated that there’s a paucity of young habitats, even less than pre-European settlement, so the proportion of post-breeding habitat to breeding habitat for forest birds here is very different from New Brunswick.
That might be why species found in both places are mostly increasing here, but not in New Brunswick. Conversely, our early-successional specialist species are mostly declining. Finding the ideal proportion for a given landscape is clearly a critical issue!”