POLEBRIDGE — A building boom has hit the North Fork of the Flathead, and for once, the neighbors are celebrating.

“They’re building a town right in our town,” Polebridge Merc owner Will Hammerquist said of the colony of cliff swallows that has claimed eminent domain on the eaves of his solar barn. “They arrived about two weeks ago. And do you see any bugs? Look at what they’ve done to the mosquitoes.”

Indeed, Polebridge feels remarkably bug-free compared to the forests of Glacier National Park just minutes to the west. The tiny birds feed on flying insects. What appear to be drunkenly random flight paths actually trace life-and-death pursuits of food on the wing.

While longtime residents along the North Fork of the Flathead River routinely resist new construction in their back of beyond, the cliff swallow complex receives no complaints. That could stem from the presence of hundreds more gourd-shaped swallow nests hidden under the namesake bridge crossing the Flathead between Polebridge and Glacier. An extreme version of the “snowbird” resident, cliff swallows migrate continentally between North and South America chasing the warm weather.

“Their range has increased greatly over the past 100 years,” GNP wildlife biologist Lisa Bate said. “With all the human structures heading north, their range has expanded north. They used to be limited to natural overhangs and canyons and cliff walls. Now look under any concrete bridge over a river, and you’ll find cliff swallows underneath.”

The home-building activity resembles a human construction site as seen in time-lapse video. The swallows sizzle around a mud puddle Hammerquist unintentionally created in another maintenance project, popping above the tall grass like seeds toasting in a frying pan. Singly or in pairs, they whiz back to the barn eave, sometimes dangling a bit of grass in a beak. With stubby, knife-pointed wings they perform a few acrobatic maneuvers before deciding which nest to work on. Then depending on whether that nest is already occupied, they either fly straight in or hover just outside with an ease that would make a Marine Harrier jump-jet pilot weep in envy.

Earlier this month, the most complete half-dozen nests clustered at the apex of the roof eave. At least a dozen more extended down each side in various states of completion. Some appeared fully rounded, while others looked like muddy half-eggshells stuck to the wood. Each nest comprises 900 and 1,200 swallow beak-fulls of mud.

Unlike their more solitary cousin barn swallows, cliff swallows live in tight colonies. The birds’ nests clump together like muddy honeycombs, and some swallows occasionally swap eggs into another mother’s nest. While the Polebridge solar barn colony numbered perhaps 40 birds, Bate said individual communities as large as 3,700 have been reported.

While it looks as old as the surrounding buildings, the solar barn actually rose in 2015 to shelter a bank of batteries fed by a line of solar panels. The electricity they generate can provide more than 90 percent of Polebridge’s power needs, from its espresso machines to its refrigerators and the sound system for its occasional music festivals.

“They’re doing their thing and we’re doing our thing,” Hammerquist said of the swallows. “It’s great to see them.”

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