For every stage of forest succession, there’s a bird species that needs it. But others are flexible, thriving in many types of forests.
The blue-gray gnatcatcher, eastern wood-pewee, great crested flycatcher, summer tanager, and white-breasted nuthatch are all associated with mature forests.
But a recent study suggests these birds are forest generalists rather than mature forest obligates.
USDA Forest Service scientist Roger Perry led the study, which was published in Forest Ecology and Management. Coauthors include retired SRS scientist Ron Thill, Julianna Jenkins of the Pacific Northwest Research Station, and Frank Thompson of the Northern Research Station.
“These five species need forests,” says Perry. “But all five responded positively to clearcuts – which were slightly modified to leave one or two big trees per acre.” Clearcuts are intensive treatments that are rarely used in the Ouachita and Ozark-St. Francis National Forests today.
Populations of forest birds have been declining since 1970. In the early 1990s, Perry and Thill began a long-term study to see how birds respond to timber harvests.
In 1993, study plots in the Ouachita and Ozark National Forests were treated with four types of timber harvest, ranging from more intensive to less intensive: clearcut, shelterwood, single-tree selection, and group selection.
Bird surveys began in 1992 and required attentive ears.
“If you can’t identify birds by calls, you really can’t do a bird survey – especially in forests,” says Perry. “It’s rare that you actually get to see the bird. They’re always calling and singing up in the trees.”
Over the course of 16 years, Perry and his colleagues returned to the plots 35 times to count the birds found in each plot. Each year, the scientists tallied and analyzed species counts. Very few other studies have examined responses of forest birds to timber harvest for such a long period of time.
Although five species benefited from all harvests, including the intensive harvests, less intensive harvests were better for the majority of the 18 bird species studied.
Twelve of the 18 species responded positively to partial harvests that retained some overstory. “Low intensity harvests, such as single-tree and group selection, benefited many species,” says Perry. “Such harvests maintained more species than more intensive harvests did.”
However, ovenbirds appear particularly susceptible to timber harvest, especially more intensive harvests such as clearcut and shelterwood. Even low intensity harvests reduced the numbers of this species. After 16 years, populations did not recover to pre-treatment levels in the more intensive harvests.
Ovenbirds, along with scarlet tanagers, seem to like thick midstories. “Ovenbirds are ground-nesting,” says Perry. “They probably like that extra shelter above their heads when they’re nesting. It probably provides some extra protection for them, some cover.”
Using a variety of methods to harvest timber produces varied habitats that maintain the full suite of forest birds, but maintaining some mature forests with well-developed midstories is also important.
“Diversity of the landscape equals diversity of birds,” says Perry.
Forest managers often manage for early successional habitats, partly for the sake of birds and other wildlife. Timber harvesting creates favorable conditions for bird species that prefer early successional habitat, as Perry and Thill found in an earlier related study.
Early successional habitat has dwindled and could explain the decades-long decline in forest birds. In some parts of the northeast U.S., you can drive for long distances through forests and see few openings, as Perry points out. Habitat loss in central South America could also be affecting Neotropical migrants, such as the red-eyed vireo and the scarlet tanager. These species spend the winter in South America and fly thousands of miles.
The current study is part of a suite of studies produced for the Ouachita National Forest. Besides forest birds, studies addressed small mammals, pine regeneration, hydrology, and visual quality.
Results from the first five years are available as a GTR titled Ouachita and Ozark Mountains Symposium: Ecosystem Management Research.
For more information, email Roger Perry at email@example.com.