Rare silver-haired bat colony discovered in Fredericton old-growth tree

Donald McAlpine was shocked when he got an email last summer with some photos of baby bats, and one question: "So what are these?"

The curator of zoology at the New Brunswick Museum was excited to identify the tiny grey mammals as silver-haired bats, and even more amazed to learn they were part of a larger colony.

"Nature is full of surprises," McAlpine said. "I was really quite floored because we just didn't expect to find this species breeding here."

Of the seven species of bats in the province, silver-haired bats are the rarest. The discovery of the "maternity roost" is the first ever in Atlantic Canada, and sets a northern record for the species in North America. 

"There are no other records of maternity roosts for the species in the region at all," said McAlpine, head of the natural history department at the museum. "In fact, there's only been a handful discovered in North America ever."

These "secretive" bats may never have been discovered had it not been for a windstorm last June that snapped the top of a 30-metre-tall eastern white pine tree, leaving a stub of about nine metres.

"It was considered a hazard by the homeowners so they had an arborist come in and remove that stub, and when it hit the ground, some of the bats flew out," McAlpine said.

The large old tree, which was part of a stand of overmature trees, was about 50 metres from the Saint John River in Fredericton's Silverwood neighbourhood. It was cut down in late June 2020.

"There was one adult female that was killed when the tree came down along with four pups, and the rest of the females flew away. So we estimate based on the number of pups that there were eight to 10 adults in that colony."

'We're hopeful they survived'

Aside from the five bats that were killed, 13 tiny pups survived. The smallest weighed just 4.7 grams and still had its eyes closed. McAlpine estimated the babies were just days or weeks old, and compared their size to that of "a thumb drive with wings." 

Wildlife biologists carefully carried the surviving pups up a ladder and placed them in a nesting box five metres off the ground in a nearby tree, hoping their mothers would return and re-establish the colony in another tree.

"They disappeared over the next couple of days," he said of the babies. "It's possible that a predator could have got them … but there was no sign when we looked in the box that there had been any massacre of baby bats, so we're hopeful they survived."

The most likely predator in the area is the red squirrel, which has a reputation, according to McAlpine, for feeding on baby birds, bird eggs and other small vertebrates. 

Few existing specimens of silver-haired pups

The bats that were killed when the tree was downed are now in the collection at the New Brunswick Museum and are among only "a half-dozen or so silver-haired bat pups in collections anywhere in the world."

McAlpine said it underlines just how little we know about the basic biology of some Maritime wildlife, and how important old-growth trees are as habitats.

"The females establish small maternity colonies in large hollow trees or under a bit of bark … typically they give birth to two young at a time, to twins," he said.

"And because they're so well concealed in these hollow trees — often in an old woodpecker nest and often quite high up — they're often not encountered."

McAlpine said there have been very few occasions where anyone has been able to look at silver-haired pups at all, let alone up close.

Silver-haired bats are believed to reach close to their adult size at about three to six weeks of age, when they begin to fly. They migrate to the southern United States in the winter.

"One thing I think that's significant is that it was in a large old tree, and large old trees are in shorter supply all the time in this part of the world," McAlpine said.

"It emphasizes the importance of maintaining old growth stands and older trees, because it's not just bats that are dependent on them, it's other species as well. But in this particular case — this particular kind of bat needs these older trees to establish these maternity colonies."

While the silver-haired bat is common in the Canadian Prairies, there have never been any reports of the species in Prince Edward Island or Newfoundland and Labrador. Until now, only four of the bats have been documented in New Brunswick or Nova Scotia since 1898, and never a breeding colony.