Deep-Woods Birds Need Young Forest
Conservationists working to reverse population declines of forest-interior songbirds have mainly tried to preserve the mature forests where those birds nest. But recent research by biologists and ornithologists suggests that young forest may be just as important for deep-woods nesters: scarlet tanagers, wood thrushes, ovenbirds, and Blackburnian and cerulean warblers, to name but a few.
Several in-depth studies have shown that after mature-forest birds nest and fledge their young, both adults and fledglings shift from older forest to young forest, including tracts that have recently been clearcut through timber harvesting. Among the densely regrowing trees and shrubs, the birds find abundant insects and fruits. These high-value foods let immature birds grow quickly and help both young and adults build up fat to fuel their southward migration. The vertical structure of the dense young trees in clearcuts appears to protect the birds from predators, such as hawks, as they go about the important tasks of growing, molting, and feeding.
Biologist Scott Stoleson works in the U.S. Forest Service’s Northern Research Station in the Allegheny National Forest in Pennsylvania. Between 2005 and 2008, on four sites on the National Forest and on private timber inholdings, Stoleson and his colleagues used mist nets to capture 3,845 songbirds in mid- and late summer, after young birds had left the nest. Half of the time, the researchers set up their nets in the understory of mature forest; the other half of the time, they placed their nets in clearcuts. After capturing birds, the scientists recorded the birds’ species, whether they were fledglings or adults, and their overall condition, including how far along the adults’ post-breeding molt had progressed and whether they’d begun building up fat deposits critically important for the upcoming migration.
Altogether, the researchers captured and evaluated birds of 46 species. Of these, 33 percent were mature-forest specialists, 22 percent were forest-edge species, and the remaining 45 percent were young-forest specialists. While individuals of all 46 species were captured in the young forest areas, only 29 species were netted in the understory of nearby mature woods.
Stoleson concluded that regenerating clearcuts – areas where mature trees had been harvested, and where young forest was growing back – attract and are heavily used by most forest-interior birds during the post-breeding season. Also, the birds appear to boost their fitness by consuming the food resources so amply present in the cuts, compared to birds that remained in deep forest settings.
Stoleson’s work implies that recent declines in the populations of forest-interior birds may stem in part from the increasing maturity and homogenization of our woodlands. He says, “Humans have really changed the nature of mature forests in the Northeast. Natural processes that once created open spaces even within mature forests, such as fire, are largely controlled, diminishing the availability of quality habitat.”
He adds, “Some young forest habitat within large areas of more-mature forest may be necessary to sustain some forest-interior bird species. Right now, the amount of young forest in the northeastern woods is at its lowest point since records began to be kept. Can this be a stressor affecting deep-forest birds, as well as the birds that breed in young forest habitat?”
It’s clear that young forest is necessary for the birds that need that thick, tangled habitat for nesting. But as research has begun to demonstrate, it may be just as vital to many mature-forest birds after young have left the nest and before adults and juveniles undertake the strenuous southward migration.