Bank swallows, a threatened species in Ontario, use their wings, beaks, feet, and determination to dig burrows two feet into sandy cliffs along lakesides and rivers to raise their young.
As these habitats are lost to land development in Southern Ontario, however, bank swallows have turned to nesting in aggregate pits. But are these human industrial sites a viable alternative?
Researchers from Trent University’s Environmental and Life Sciences program think they are. They examined whether bank swallows were able to successfully raise young in aggregate pits near Peterborough and along the northern shore of Lake Ontario. The findings show that aggregate pits should be managed to promote the survival of these nesting birds.
The research project was a master’s thesis by Tianna Burke, currently a conservation biologist with the Georgian Bay Biosphere Reserve. Burke describes bank swallows as mesmerizing to watch.
They are tiny birds, weighing about as much as six pennies (if you can remember pennies). Along with a brown upper body, bank swallows have a dark “necklace” across their white belly. Their flight is “really fun, it’s erratic,” says Burke, and the birds produce a distinctively raspy and chattery call.
Bank swallows nest in tall, vertical sandy cliffs along lakeshore and rivers. They dig burrows into the side of the banks for nesting: no small feat for such a tiny bird. Burke says that the swallow starts its burrow by “clinging to the bank with its feet and digging with its bill and wings.” Once a burrow starts to take shape, the bird uses its feet to kick out dirt.
Unfortunately, the bank swallow’s preferred nest sites also happen to be areas where humans like to build. “There have been a lot of erosion control measures put up on these cliffs,” says Burke, “which is great for people whose homes are on them, but it also stops bank swallows from being able to nest there.”
Burke compared nests in aggregate pits and natural lakeshore sites. She looked at how many eggs the birds laid in individual nests, how many of those eggs hatched, and whether the babies left the nest, a bird graduation ceremony known as fledging.
She found that nests were less likely to survive and produce fledglings in aggregate pits. “Banks in gravel pits can be less stable, and so these birds would burrow into it, and after a couple of weeks the bank would slump and wipe out all the nests,” says Burke. The low cliff heights in aggregate pits also made the nests vulnerable to mammalian predators such as raccoons.
Despite the dangers for barn swallows of nesting in a gravel pit, Burke found that the ones with nests that did survive actually raised more young per nest than in natural lakeshore habitats. The results were a high-stakes gamble: nests “either survived and raised more young, or they were completely wiped out.”
Another interesting finding by Burke and her team was that the adult swallows in aggregate pits weighed significantly less than adults on the lakeshore by the end of the nesting season.
“The adults on the lakeshore are able to provide for their young and provide for themselves throughout the whole year. In aggregate pits, they’re not able to do that. They can provide for the young, but the adults themselves are not able to maintain their masses.” This poses a problem for adults when winter rolls around and it’s time for the birds to migrate south.
The study led Burke to working with aggregate pit operators to develop management plans to help bank swallows successfully nest and fledge in gravel pits. Plans included leaving certain pits for birds to nest in throughout a year, and preventing swallows from building holes in active extraction pits.
Burke encourages cottagers and other landowners to help protect bank swallows. If you purchase gravel directly from an aggregate pit, make a point to ask if bank swallows are nesting at the pit, and if they are, how the site is managing them. And if you live close to an aggregate pit, help feed bank swallows by providing a buffet of insects.
“Bank swallows don’t necessarily feed only where they’re nesting,” says Burke. “They have about a kilometre range that they will collect insects from to bring back to their nests.” Landowners can promote healthy insect populations by preserving natural spaces on their property and by refraining from using pesticides and herbicides.