Some Birds Gain, Others Lose From Logging

When more than half of trees are harvested from a mature forest, the populations of some bird species plummet.

An analysis of 42 studies looking at how partial harvesting of forests affects 34 bird species finds that 14 species become less common after a site is logged.

Meanwhile, another six species gain abundance after cutting a forest.

The amount of trees left after harvesting has a bearing on how drastically bird numbers drop. Overall, when 70 percent of the trees are left, a few species decline by 25 percent, but no populations fall by as much as 50 percent. It's when more than half the trees are logged in an area that severe drops of 75 percent or more occur for some species.

Certain species are far more sensitive than others to tree removal. Two birds particularly require the habitat only found in intact mature forests. Brown creepers use the bark of large trees for feeding and nesting. Ovenbirds need deep leaf litter under a closed tree canopy. In contrast, other species decline by as little as 25 percent when three-quarters of the trees are harvested.

Birds that do benefit from tree harvesting respond enthusiastically, generally increasing by at least 50 percent in number when 70 percent of a forest is retained.

The meta-analysis only covered studies examining partial retention harvesting that left behind a uniform distribution of live trees. The research came from many different regions and forest types including Douglas-fir forests in British Columbia and Oregon, ponderosa pine in Arizona, beech in West Virginia and New York, and sugar maple in Ontario and Michigan.

The analysis found that most kinds of birds respond consistently throughout Canada and United States. Three species though, black-throated green warblers, rose-breasted grosbeaks and yellow-rumped warblers, are more sensitive to tree harvesting in boreal forests than in habitat farther south.

Birds that become more abundant after partial forest harvesting: American Robin, Brown-headed Cowbird, Chipping Sparrow, Dark-eyed Junco, Indigo Bunting, White-throated Sparrow

Birds that decline in number after partial forest harvesting: American Redstart, Black-throated Green Warbler, Brown Creeper, Golden-crowned Kinglet, Hermit Thrush, Least Flycatcher, Ovenbird, Pacific-slope Flycatcher, Red-breasted Nuthatch, Red-eyed Vireo, Rose-breasted Grosbeak, Swainson's Thrush, Winter Wren, Yellow-rumped Warbler