The first phase in a two-year study of northern long-eared bats on the Vineyard has shed light on roosting behavior, but left unanswered the key question of where the federally-listed bats, which have all but disappeared on the mainland, spend the winter.

White-nose syndrome has killed up to 99 per cent of the northern long-eared bats in the Northeast. But the Vineyard population does not show signs of the disease, leading biologists to believe those bats are not leaving the Island, but staying in cellars, cisterns or other cave-like places during the winter.

Working with the United States Fish and Wildlife Service last year, BiodiversityWorks in Vineyard Haven captured bats and confirmed that the females on the Island were breeding. The fish and wildlife service has since declared the bats a threatened species under the Endangered Species Act, with a final ruling on conservation measures expected this winter.

A female northern long-eared bat, weighing in at 9.3 grams. — Ivy Ashe

The two-year study began in June with funding from the Martha’s Vineyard Vision Fellowship. It aims to study maternity roosts and track the bats to their winter homes. Beginning in June, the organization worked with landowners, along with the Vineyard Open Land Foundation and The Nature Conservancy, to capture bats, tag them with tiny radio transmitters, and follow them to their roosting sites.

It was the first-ever bat telemetry study on the Vineyard, and revealed some surprising facts about the species. About 30 per cent of the bats that were observed roosted in houses, just under the trim board of the roof, possibly drawn to heat from the sun.

“They are that tiny,” BiodiversityWorks director Luanne Johnson said of the bats, which can fit inside a closed fist. “They can just fit right in that gap between the shingles and the trim board, and they just hang up there and you can’t even see them.”

Homeowners had no idea the bats were there, but most were accepting when they found out. The bats caused no damage, Ms. Johnson said. And although all mammals can carry rabies, bats rarely attack people.

Other roosting sites included trees with knots, holes and other cavities. One way people can support northern long-eared bats is to not remove all of the dead trees on their properties, Ms. Johnson said, especially during the roosting season.

Northern long-eared bats are active from late April through October, usually roosting between May and July. They can live up to 18 years, but females produce only one pup per season, which makes the species especially vulnerable to white-nose syndrome, since they cannot replace themselves quickly enough to absorb the losses.

BiodiversityWorks tagged 12 bats this summer, including nine females near Lambert’s Cove, one near Edgartown Great Pond, and a recently-flying male in Oak Bluffs. “So we know there are at least three viable maternity colonies on Martha’s Vineyard,” Ms. Johnson said, adding that there are likely others.

Two bats that were tagged in late summer have not returned to their hibernating spots, but neither have they left the Island. “If they were going to be somewhere on the mainland for the winter they probably should have migrated by now,” Ms. Johnson said. “They probably aren’t going anywhere.”

As of this week, only one bat had not lost its tag, and was living in a tree in Edgartown.

Without any documentary evidence of hibernating bats, researchers can’t say for sure whether the Island is free of white-nosed syndrome, which is caused by a fungus that thrives in cool, dark places and only affects bats in hibernation. But none of the bats captured so far have shown signs of wing scarring, which often results from the disease.

Scientists who study bats have focused on caves and mines, which do not exist on the Vineyard, so local researchers have no data related to where the bats hibernate on the Island. Construction workers have reported seeing bats in houses in the summer, but no bats have been reported in recent years in the dead of winter.

Ms. Johnson and others will continue digging through their data and looking for roosting sites, and compare notes with the Northeastern Bat Working Group, which draws together a range of people interested in bat research, management and conservation. Field work will pick up again in the spring. But next year the team plans to tag later in the season to improve their chances of finding the overwintering sites. Usually the nanotags fall off after just a few days.

“Wherever that hibernacula [hibernating site] is on Martha’s Vineyard, it must be pretty safe because the bats have survived this long,” Ms. Johnson said. She believed it could be a single site or several sites around the Island.

“So where is that? No one knows.”