Jan. 31, 2011, 10:59 a.m.
Every evening, across the Lower Mainland, thousands of crows are seen crossing the skies at dusk, heading towards Burnaby's Still Creek area to roost.
Whether you love them or hate them, this ubiquitous urban species thrives in the conditions we create, and the health of their populations says a lot about how we treat our environment. Jennifer Moreau takes a closer look one of the Lower Mainland's most spectacular natural phenomenon.
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On the gravelly side of the road in Burnaby's Still Creek business area, a handful of volunteer bird enthusiasts is wrapping up the seasonal Christmas Bird Count, with their heads turned towards the cold, dimming sky. The annual census of winter birds is finishing at the site of the largest and most spectacular local display of bird behaviour - thousands upon thousands of crows coming home to roost at dusk.
The crows gather a few blocks from Still Creek. Most flock to trees and rooftops, some hop or waddle along the ground and others bob their heads while cawing from branches and wires that bow beneath their weight.
George Clulow, longtime organizer of the local count, spots an intruder crossing the open sky.
"Cooper's! Cooper's hawk right there," he shouts, peering through a set of binoculars. The birdwatchers chuckle at the hawk's misfortune as an angry black swarm amasses, chasing after the predator. The hawk, which may normally make a meal out of a crow, is outnumbered by about 100 to one.
"They'll give him heck," Clulow says. "Wrong place to fly through."
The crows continue gathering, and as the light fades, the restlessness grows. Then suddenly - thousands of birds take flight for the creek, and from below the sky seems to spin. With all the droppings, it sounds like rain, and Clulow notes that women who work in the area have been known to walk home with parasols.
We head along the Central Valley Greenway, following their flight path, into the heart of the roost. It's dark now, but the sky is clear and moonlight fills their sleeping quarters. Every tree is full of crows, cooing and clucking as they settle down for the night. They rest till morning and depart once the light returns, with the same raucous pageantry.
According to this year's bird count, 23,000 crows settle at the Still Creek roost, just a few minutes' walk from the Gilmore SkyTrain station. It's the biggest roost in the Lower Mainland, drawing crows from miles around. They seem to prefer the trees along a small stretch of Still Creek, but many settle on rooftops and on telephone wires. In spring, the numbers drop as the adults leave to mate and build nests. The roost has been around Still Creek for as long as anyone can remember - at least 20 years, likely more.
Are the numbers declining?
According to the count, numbers for the roost appear to be declining, although Clulow is reluctant to make that assertion. The tally is a rough estimate at best, and it's a bit tricky to count thousands of crows in flight.
In 2001, there were 10,000, and the numbers fluctuated over the years, peaking in 2009 with 30,000 and dropping to 23,000 in 2010.
Michael Jackson of Culex Environmental has studied the local roost because health officials had concerns about West Nile virus. (It turned out there was no health threat.)
Jackson suspects the drop in numbers could be linked to destruction of habitat in 2006. The crows used to sleep to the east of where they are now - that was until the area was razed for development.
"They were just demolishing the roost to make way for Costco and the Keg restaurant there," Jackson says. "They were highly stressed out when they lost that roost. They were flying all through the night, getting agitated, with no place to settle."
Crows don't fly well in the dark, and they were bumping into each other. "It was pretty crazy. It was quite obvious they were pretty unsettled and agitated."
Jackson surmised the stress may have impacted mortality rates or driven some to other roosts. But, he adds, they are very adaptable.
So adaptable in fact, that they've squeezed their new roost to the aforementioned stretch along Still Creek.
Clulow has also noticed the roost moving around over the years.
"They seem to have always been along the Still Creek corridor, but in the last seven to eight years they've been moved around more," Clulow says.
Even with the loss of habitat, they stick to the same area and adapt in situ, he says.
"(But) continuing development has forced the birds into areas where people don't want them. It not the birds' fault. Their normal habitat has been built over. There's now quite a restricted area they've been forced into, and they are moving into the buildings and the street trees."
Why do they settle in Burnaby?
While it's clear the roost has been pushed around, the crows adapt regardless.
Researcher John Marzluff says crows do exceptionally well with people. The University of Washington professor and author is an expert on all things related to corvids.
"Most birds have a difficult time surviving with us, but crows are one of the few species that really thrives on the way we change the environment," he says. Sensitive species would not use the urban environment to the extent that crows do.
"(Crows) are able to use the structures we provide, either our buildings or our wires and power lines, ... and they are able to use the way we reconfigure the landscape."
Marzluff says crows benefit from human presence, since they like a mixture of few trees, lots of open lawns and plenty of garbage.
"We've caused a population boom. So these large, urban roosts are where the crows are spending the night so they can exploit the urban landscape during the day," he says. "These birds are actually drawn in from other areas to these roosts, they are drawing birds in from the countryside into these urban areas."
And there's a simple reason they roost in such large groups, Marzluff points out.
"It's safety in numbers, and they are reducing commuting costs. So, (as) they are going to similar places to feed, they are going to roost in a centrally safe location so they minimize their commuting costs and maximize anti-predator defense," he says.
They could get eaten by owls if they roost on their own, but if you're nestled in a mob of 10,000, your chances of getting picked off decrease. And they probably settle in Burnaby because it's centrally located, he adds.
"They pick a spot that is relatively central or near to where they are all feeding. So some might go north, some might go south, but they are all going about the same distance," Marzluff says.
Despite the reduced habitat, development in the Still Creek area could actually be creating conditions crows favour by reducing the number of hawks and owls.
"Owls, especially, eat them at night in those roosts - that's the most dangerous thing to a crow. My guess is that maybe because the landscape has been denuded around there, there's still enough space for them to perch, but there's basically a reduction in the predator load there, so again they take advantage of what we've done," Marzluff says.
Another reason crows are so adaptable is intelligence. They come from the family of corvids, which includes ravens, magpies and jays. Corvids have relatively large brains for their bodies, proportionally comparable to small primates. In a research experiment featured on The Nature of Things, Marzluff found that crows were able to identify threatening characters by recognizing human faces. Participants wore masks when capturing and tagging crows, and once released, the crows could pick the masked faces out of a crowd and would warn other crows with alarming calls.
Some crows, especially the New Caledonian, can create and use tools to get food. There are legendary reports of crows holding funerals for their dead, where they perch in silence, looking over the lifeless body. They are also monogamous, usually mating for life, and young crows help their parents raise the next generation, something relatively unusual among birds. They look out for each other and have organized social groups. In a way, they are like us.
"I have a great deal of respect and admiration for crows. They are very intelligent," says Clulow. "There's a social structure in crow society. Behaviourly, they are very interesting. They accumulate in these huge spectacular roosts and actually are providing some urban clean-up service. ... That's why I think they are admirable birds."
Despite their intelligence, many abhor them. In European culture especially, crows are perceived as bad, Marzluff says. They are black, associated with death and thought of as bad omens.
"That turns a lot of people off to them initially," he says. "And they see them eating bird eggs or a baby duck, or eating garbage. They get a lot of bad press for what they do. So that polarizes some people against them."
The health of crow populations mirror a lot of the damage we do to the environment, Marzluff says. The more garbage we produce and the more we change the natural dense land covers, the more crows thrive.
"We changed what was thick, dark forest, where there were no crows at all, into a mixture of farmland and sports fields and subdivisions, which is perfect for crows. They are mirroring our change of the landscape," Marzluff says. "So when people are saying they don't like crows, they are really saying they don't like the way I act in the environment. That's why they might see a crow eating a baby bird, but the only reason that crow is eating a baby bird is because we changed the landscape in such a way that we promote crows. I think you do need to question yourself when you say that, and it's not the crow. The crow is just mirroring what we've done."
For more on crows, see Jennifer Moreau's blog, Community Conversations, at www.burnabynow.com.