The most dramatic declines in bird populations were seen among those that live off flying insects, scientists say. Known as aerial insectivores, this diverse group of birds has declined 45 percent in Vermont, according to the study.
The study focused on 11 species of aerial insectivores that were among the 13 species of Vermont birds found to have undergone the most serious declines, said Steve Faccio, one of the study’s authors and the co-founder of the Vermont Center for Ecostudies.
This group of birds includes the tree swallow, the yellow-bellied flycatcher, the chimney swift and the eastern wood pewee.
This dropoff in insect eaters mirrors a trend seen nationwide in recent decades, Faccio said.
These birds’ food — aerial insects — is what the species in this highly varied group have in common, leading scientists to suspect that some widespread trend among flying insects is what’s driving the decline in birds that eat them, Faccio said.
Scientists say the decline might also be attributable to polarized light pollution and to a possible recent mismatch in timing between important points in the birds’ and the insects’ life cycles, Faccio said.
Natural polarized light is found most often reflected off the surface of water, but a variety of artificial surfaces reflect polarized light as well, such as windows, painted automobiles and even black plastic sheeting, according to the study. Because many insects are drawn to polarized light, scientists believe that some of them confuse highways for rivers, and that similar mistakes on a larger scale may be among the drivers of a decline in aerial insect populations.Mismatched timing between species can occur when global warming or other climatological occurrences lead insects to hatch earlier than normal, Faccio said. If they’re hatched and flying before the birds that eat them are active, aerial insectivores may be running out of food simply because of a timing issue, he said.
But there’s not enough information about insects yet to say whether their numbers have declined across the board, Faccio said. Population trends are known for some insect species, but aerial insectivores live off thousands of different species of insects, and it’s still unknown whether the diminishing population of aerial insectivores is matched by a drop in their food source, he said.
Vermont’s wind turbines are not thought to have any appreciable effect on the bird populations studied, Faccio said.
Other factors are thought to have contributed to the decline in bird numbers, said Faccio. These include pesticides and loss of habitat due to development.
Scientists have identified a number of potential causes for the decline, but there’s no consensus yet as to which is most responsible, Faccio said.
Although the number of aerial insectivores in Vermont appears to have dropped significantly, other groups of birds were found to have grown in number, according to the study.
Those include ground gleaners, a type of bird that plucks insects from the ground, such as the hermit thrush, the yellow-shafted flicker and the ovenbird. Another is the high-canopy foragers, including the bay-breasted warbler, the cerulean warbler and the scarlet tanager. Those groups grew by 22 percent and 11 percent over the 25-year study’s duration.
The study includes 125 species detected at about 30 study sites around Vermont, Faccio said. Each location contains five predesignated sites where volunteers and VCE staffers tallied how many and what type of birds could be seen or heard over a 10-minute period, he said.
For the first five years after the study began, researchers saw on average 14.8 individual birds at each of these sites during a single 10-minute observation period. For the five years leading to 2013, the last year the study concerns, researchers found an average of 12.7 individual birds per site — a 14.2 percent decrease.
The decrease occurred mainly during the first half of the 25-year period covered in the study, Faccio said.