Bat boxes were supposed to be a safe haven for bats. So why are they killing them?

At ƛ̓éxətəm Regional Park in Port Coquitlam, just outside Vancouver, sits an unusual housing complex of tall wooden structures, with narrow little openings.

The sole tenants? Roughly 800 female bats.

Mothers, daughters and grandmothers return to the bat boxes summer after summer to give birth and raise the young. For more than a decade, bat enthusiast John Saremba has helped care for the colony, maintaining the artificial roosts designed to shelter the animals from environmental threats.

“They’re incredibly tender, wonderful mothers,” Saremba told CBC’s What on Earth? host, Johanna Wagstaffe. “In fact, if a female bat is having problems with pregnancy, the other bats will rub her tummy.”

That show of tenderness made his discovery in July 2018 all the more heartbreaking; he arrived at the boxes for a routine check and found 65 dead bats.

“We came out here and I was just devastated,” he said. “I really didn’t know what to do.”

The very boxes meant to protect the bats can become deadly, scientists say, as climate change fuels more frequent and intense heat waves. The boxes can then trap heat inside, exposing the animals to extreme temperatures. 

That doesn’t mean we should abandon bat boxes — bats are already under threat due to disease and habitat loss — but as global temperatures keep rising, conservationists say the boxes need a few tweaks. 

What exactly happened in 2018?

It wasn’t the first time the colony experienced dangerously high temperatures; the bats had survived a heatwave a month before the fatalities. So what was different this time? And if conditions had become unbearable, why didn’t they simply fly away?

Leah Rensel, a wildlife biology PhD candidate at the University of Montana, was studying the Port Coquitlam bats when the die-off occurred. 

“The temperatures inside the bat boxes had exceeded, I believe it’s 44 C, which is lethal for the species,” Rensel said.

Rensel suspects the bats’ fiercely protective maternal instincts was the reason they stayed, noting the colony was likely going through parturition in July 2018, a time when the female bats gather to give birth. 

“[The babies] are really small … so the bats had these vulnerable pups and they didn’t want to leave the roost, even though the conditions were too hot.”

Though expectant mothers seek out warm spots — so they can devote themselves to giving birth, nursing their young and hunting insects instead of temperature regulation, there’s a limit to how much heat they can handle.

“I call it the Goldilocks zone,” Rensel said, which falls between 35 C and 41 C for the little brown bat and the Yuma Myotis bat. 

“So it’s not too hot and it’s not too cold … so if you’re a bat in a maternity colony and you find a roost that hits that most of the time, that’s golden.”

But these little wooden boxes can heat up quickly, particularly when they’re painted black and placed in direct sunlight, as the Port Coquitlam boxes were.

A cluster of bat boxes housing female Little Brown and Yuma Myotis bats in Səhətləm Regional Park in Port Coquitlam. (Submitted by John Saremba)

And the combination of higher temperatures and the stuffy conditions inside the boxes led to humidity, which is especially harmful to the inhabitants. 

“What climate change is doing is essentially increasing the temperature component of their microclimate,” said Cori Lausen, director of bat conservation with the Wildlife Conservation Society Canada and Rensel’s thesis adviser. 

“When you bring temperature and humidity together, that’s heat stress,” she said, noting that bats don’t sweat, which makes it harder for them to cool down.

Why bats are in danger

Bats are under threat from more than just heat.

Worldwide, habitat loss and forest fragmentation have reduced the availability of natural roosting sites and the insect-rich habitats bats rely on for food. In North America, the fungal disease known as white-nose syndrome has decimated their populations.

Of the world’s nearly 1,400 known bat species, 25 are considered critically endangered, 87 endangered and 110 vulnerable, according to the International Union for Conservation of Nature. 

A world without, or with significantly fewer, bats could devastate ecosystems, because the animals perform natural pest control, suppressing insect populations that can ravage forests or farmers’ fields, according to a 2019 paper published in the Biodiversity and Conversation journal.

In tropical and subtropical forests, bats disperse seeds through their droppings, an important role in regeneration. 

And the consequences of losing these fanged creatures may even extend further: one peer-reviewed study even found that where bat populations died off, pesticide use jumped — and that, in turn, increased infant mortality rates. 

WATCH | Bats have a housing crisis. And it’s getting worse :

Bats have a housing crisis. And it’s getting worse

Sixty-five bats were found dead inside a home built to protect them. What went wrong? A surprising bat housing crisis reveals how a warming climate is changing the places bats call home. From devoted bat moms, to dangerously overheated bat boxes, and some equally surprising solutions — including bat condos with emergency exits and artificial tree bark.

So, what’s the solution?

The answer, Lausen says, is not to scrap bat boxes but to use more of them.

“Never take away a box, never change a box,” she said. “Add boxes around your property, for example, to give them more options.”

She recommends placing boxes less than 100 metres apart so that bats can easily switch roosts if they need to warm up or cool down.

“Get a nice collection of bat boxes going: different sizes, different styles, different solar radiation.”

That could include boxes with different types of venting or different paint colours, which can affect how much heat the bats absorb, Lausen says. She adds that an “absolute minimum” of three bat boxes should be installed on each property.

Another option is a “bat condo,” Lausen said, complete with different spaces, so the animals can move between microclimates without having to leave the roost, much like an old attic where city bats traditionally made their homes.

The first bat “mini-condo” designed to hold 1,000 bats built in a regional park in Metro Vancouver. In a bat condo, bats can move between different microclimates without having to leave the roost. (Submitted by John Saremba)

In B.C., Saremba has been experimenting with the old bat houses and making new ones. He’s added shading to the old boxes using white blankets and building emergency exits out of PVC pipes. 

Following those changes, bat deaths stopped.

And he’s also built a mini condo designed to mimic an attic, giving bats more room, more temperatures and humidity levels to choose from.

It even includes somewhere for the youngsters to practise flying.

“It’s like a teenager learning to drive,” Saremba explained. “You don’t go right on the highway first thing, no, you go on the side roads.”