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 Vermont forest birds over the past 25 years. While not all of the 125 species included in the report are declining – some are even increasing – the researchers say that the overall state of forest birds in the region raises concerns about both birds and forests.
“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.”
The data were collected by highly skilled birders who, beginning in 1989, surveyed 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 10 years of the study. After then stabilizing for a number of years, overall bird numbers began decreasing again in 2008.
An in-depth analysis of 34 of the most widely distributed bird species found that 13 have experienced significant declines, including the Canada warbler, white-throated sparrow, great crested flycatcher, and veery. Populations of just eight species have increased – American robin, red-eyed vireo, and ovenbird among them.
Faccio said that forest fragmentation, climate change, invasive species, and threats on the birds’ wintering grounds could all be factors contributing 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 by the natural progression of forest growth.
The birds that fared the worst in the study are the “aerial insectivores” – birds that catch and eat insects on the wing – such as the 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 causes such as 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 people 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.