In one of the longest-running studies of its kind in North America, the Vermont Center for Ecostudies has documented a 14.2 percent decrease in forest birds in Vermont over the past 25 years. While not all of the 125 species included in the report are declining – and some are even increasing – the researchers say that the overall state of forest birds in the region raises critical concerns about birds and forests alike.
Unlike the North American Breeding Bird Survey, which has conducted an annual assessment of bird populations along roadsides since 1966, the Vermont study provides data about birds in interior forests away from the effects of roads.
“Forests are vital to our economy in Vermont, and birds are vital to the health of the
forest,” said biologist Steve
Faccio, the lead author of the report. “This should serve as a wake-up call to
focus our efforts on maintaining healthy forests and thinking about how we
should do that.”
|Canada warbler (Garth McElroy)|
The study used highly-skilled birders who traveled 31 forested transects twice during each breeding season and counted all of the birds they saw or heard at five sites along each transect. Most of the population declines they observed occurred during the first ten years of the study. After stabilizing for about a decade, bird numbers began decreasing again in 2008.
An in-depth analysis of 34 of the most widely distributed species found that 13 have experienced significant declines, including Canada warbler, white-throated sparrow, great crested flycatcher and veery. Populations of just eight species have increased, among them American robin, red-eyed vireo and ovenbird.
Faccio said that forest fragmentation, climate change, invasive species and threats on the birds’ wintering grounds could all be contributing factors to their decline. “Just because we’re seeing lower populations here doesn’t necessarily mean it’s something happening on the breeding grounds that’s causing the decline,” he said.
Changes in habitat due to maturing forests could explain some of the declines. Habitat for species that nest or feed in the lower or middle canopy layers, for instance, could be affected through the natural progression of forest growth.
The birds that fared the worst in the study are the “aerial insectivores,” those that catch and eat insects on the wing, such as Eastern phoebe, Eastern wood pewee and least flycatcher. The 11 species in this group declined by 45 percent.
“That leads us to believe that there’s something going on with their prey, probably a combination of effects like pesticide use, changing climate and habitat,” Faccio said. “Polarized light pollution is having a devastating effect on broad groups of insects, which could lead to reproductive failure of some water-breeding insects.”
The report recommends that those interested in managing their forests for birds should consider creating more structural diversity to emulate natural disturbances in mature forests, while also retaining a high proportion of large trees to support canopy and cavity nesters. Land managers should also focus on protecting uncommon forest types, contiguous forest blocks of more than 250 acres, and corridors that connect conservation areas.
This article first appeared in the fall 2017 issue of Northern Woodlands magazine.