Boreal bank swallows: suddenly a hot topic

It often requires an informed, enthusiastic visitor to remind us not to take the "wild" in our Wilderness City for granted.

It often requires an informed, enthusiastic visitor to remind us not to take the “wild” in our Wilderness City for granted. “In Whitehorse you guys are really lucky because the Yukon River runs right by the city and you get to see bank swallows flying in and out of their burrows in the river bank,” says Ontario environmental biologist Sonje Bols.

Bols, a graduate student at Nipissing University in North Bay, shared some of our luck – and her enthusiasm – in Whitehorse last summer. She spent six weeks here studying bank swallows.

For the biologist, who had only seen these particular swallows as occasional members of mixed migratory flocks passing through northern Ontario, the Yukon experience was a big, informative treat.

“We were studying colonies that were right in downtown Whitehorse,” she says. She and her Yukon College student assistant would embark on the river from Shipyards Park and immediately encounter bank swallows – small grey birds with bright-white undersides, marked by a distinctive grey-brown chevron.

Bank swallows need all the attention they can get. Between the 1970s and 2011, their population fell more than 90 per cent, Bols says. By 2013 they were listed as a threatened species by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). One of the results of the decline was that Bols, who was looking for a thesis topic for a master’s degree in environmental studies, was urged by her project supervisors to devote her efforts to the beleaguered bird.

While bank swallows were under-researched as a whole, boreal bank swallows have been even more seriously overlooked.

“As soon as a species is put on the species at risk list, right away there’s a big rush to find out as much as we can about that animal,” Bols says. “That’s because as soon as you list an animal as a species at risk, you have to develop a kind of recovery strategy to bring populations back up to a healthy level.”

And the initiative is not just about bank swallows. These birds (Riparia riparia) are part of a bigger, even more disturbing picture. Bank swallows are aerial insectivores: they grab their insect suppers on the wing. Other aerial insectivores include common nighthawks, whippoorwills, tree swallows, violet-green swallows and flycatchers like the western wood peewee. All appear hard-pressed by something, somewhere in their travels. Habitat loss? Contaminants? Food shortages?

Bank swallows are “pretty unique among swallows,” says Bols. They are the smallest swallows but build large, sometimes metre-deep, burrows in clay banks.

Southern Yukon is well-endowed with welcoming clay banks. “It also happens to have a really good population of bank swallows, so I can get a nice sample size by visiting lots of colonies in one area,” the scientist says.

“What I did was canoe up and down the Yukon River from Marsh Lake to Takhini Bridge.” Once she’d located nesting colonies, she video-recorded them to see how often adults brought food to the nestlings, and thus how readily those adults were able to access food in the area.

A few weeks into the nesting season, she videoed the burrows again to determine how many of the babies survived to become juveniles. These juveniles can be readily seen and easily counted as they hang around burrow entrances waiting to be fed.

While canoeing from colony to colony, Bols took soil samples, measured bank height, counted the number of burrows, and determined which direction the burrows were facing to see if that could have an impact on swallow parenting success.

These little birds, and their eggs, have some natural predators, of course – kestrels, red squirrels, ravens, foxes – but for the most part the nests are high up sharp cliff faces and harder for predators to reach.

Over a 50-kilometre stretch of river Bols found 63 colonies, 30 of which were occupied. For a non-scientist that percentage conjures alarming images of avian ghost towns. However, 30 out of 63 is pretty normal for bank swallows, says Bols.

The low occupancy rate can be blamed, in part, on mites. The tiny arachnids are a vicious nuisance – infesting nestlings, sucking their blood and causing exhausting, effort-wasting scratching at a time when baby birds need all their energy to grow. Burrows will be overrun with mites if continuously inhabited summer after summer.

Fortunately for the birds, while mites proliferate, bank burrows themselves provide a control mechanism. They are meant to be ephemeral. Dug into steep clay cliffs, they’re subject to erosion. Wind and heavy rain will sheer the face off cliffs and erase old burrows.

When the mite-infested burrows are washed away, the parents move to old long-vacant ones or dig new ones deep in the freshly scoured cliff face.

Bols says she has about a hundred hours of videos to watch yet this semester, and will probably still be recording data from them as Christmas nears. As for the bank swallows on the videos, they’ll be spending their winter in Central and South America, perhaps dreaming of fat Whitehorse insects.

They leave for the south around this time of year – in late summer and early fall – but they’ll return to our clay cliffs in late May or early June. By then, Bols hopes to be closer to discovering what’s causing the population of bank swallows and other boreal insectivores to plummet. That will be a good, firm, first step toward reversing yet another alarming environmental trend.

This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.yukoncollege.yk.ca/research/publications/your_yukon

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