Franklin's gulls have arrived in Alberta for the summer from South America, gathering in the thousands along the Bow River. (Dave Breckon)Calgary's bug-rich Bow River has become a temporary feasting point for thousands of gulls from Chile and Peru as they prepare to nest in Alberta's lakes for the summer.

The striking Franklin's gulls — with their black heads, white bodies and red beaks — are congregating in the city's southeast edge, taking advantage of the river's supply of insects.

Every year the birds fly along the same path from their winter homes in South America, said naturalist Gus Yaki, who leads birdwatching groups to see them.

The gulls don't tend to fly in flocks, but when they arrive in Calgary they gather along the Bow River, he added.

"Right now on the river we have a hatch of chironomids — so-called midges — and other small aquatic insects, so the gulls are taking advantage of that hatching period that is occurring right now. So that's why they are accumulating," Yaki said.

"You'll see them floating down and then after 300 metres or so they fly back up river and repeat the process continuously — a never-ending ribbon of gulls floating down and flying back up."

Soon the birds will leave Calgary for lakes as far away as northern Alberta to build floating nests for breeding, Yaki said.

"At Frank Lake just outside of High River a few years ago, they estimated 75,000 pairs were present there. And that makes it probably one of the highest-populated sites on the continent," he said.

By August the gulls will head back to South America for the winter.