Each spring, the stand of cottonwood trees straddling the mouth of the Coquitlam River, where it meets the Fraser, comes alive with the honking sound of more than 200 great blue herons feathering their nests in preparation for their soon-to-be hatched young.
This year, however, all but one or two of the hundred-plus nests are empty, and silent.
Elaine Golds of the Burke Mountain Naturalists and Liz Thunstrom, chair of the Colony Farm Park Association's wildlife committee, suspect the herons' sudden disappearance may have something to do with nearby work on the Port Mann bridge.
"We think it's noise from the bridge work but, of course, there's no way of knowing that for sure," Golds said. "But we've been getting lots of reports from people who live nearby about the noise from bridge construction. I think [the herons] just decided they can't take that noise anymore."
The heron nests at Colony Farm Regional Park are high up in the trees on the south side of the Mary Hill Bypass, just east of the Coquitlam River bridge. The traffic makes it a fairly noisy spot but Golds said it's much like white noise for the herons, which have been nesting in that location for close to 20 years.
Significant vibrations and heavy machinery noise from building the new Port Mann bridge, and taking down the old one, could be enough to scare off the herons, Golds added, since they're extremely sensitive to disturbances near their nests.
Great blue herons are a common sight for visitors to Colony Farm Regional Park (as well as other heron nesting sites in Point Roberts, Stanley Park and Chilliwack) but their numbers are decreasing in other areas of the province, making them a species at-risk in B.C.
Each March, the herons start to gather in Colony Farm's expansive fields because "they need to get together and stare at each other for hours," Golds said. "They're like shy teenagers at their first high school dance."
Their numbers continue to grow until early April, when the herons head up to their nests to lay eggs that then hatch in June.
"Usually, we would see them flying around at this point, bringing sticks in to repair their nests," said Thunstrom as she walked along a wooded path in the provincially protected wildlife management area south of the Mary Hill Bypass, using binoculars to search for herons in the nests across the Coquitlam River.
But volunteer heron watchers, who count the birds daily from October through April, started noticing the herons weren't gathering in large groups as they usually do. Last week, Thunstrom said she counted just three herons in their nests, with the hope that one of their partners was off hunting at the time.
It's possible the colony has shifted to another site but Golds has investigated likely sites on Douglas Island, on the Surrey border of the Fraser and along the Pitt River dikes, and didn't see any herons.
"We're missing 200 herons," Golds said. "We're very sad, and this is a significant loss in terms of the heron population."
A heron monitoring program has been in place since work on the bridge started in 2009, said Robin Taylor, environmental manager for the Transportation Investment Corporation. This year, monitors recorded the typical nest-building and territorial behaviour common for early spring but found the herons weren't up in their nests at the start of April.
"They've been spotted flying downstream but exactly where they're going it's not known," Taylor said.
She said it's possible the herons have relocated and may return to their original location next year, adding the PMH1 wildlife monitor suspects a new eagle's nest just upstream from the bridge may also be the cause of the herons' sudden departure from Colony Farm.
"[The eagle] has been responsible for quite a bit of harassment of the adults as they've been trying to nest," Taylor said.
Monitoring over the past four years of bridge construction has shown heron numbers have stayed "relatively stable," Taylor said, adding last year a new nest was added even closer to the bridge.
Golds and Thunstrom are hoping people will keep their eyes out for the herons, and anyone who spots an unusual increase in the size of a nearby heron population is asked to call Golds at 604-937-3483 or Thunstrom at 604-939-9571.
