Counting a murder (of crows) in Langley

Residents in south Aldergrove may have witnessed a nightly murder in their neighbourhood for the last four months—or at least heard one.

The birds gather at dusk during the wintertime to sleep for safety. The sound of thousands of crows flying to their winter roost is incredibly loud. The seasonal migration is unremarkable and crow-watching is hardly a desirable activity—they’re ubiquitous year-round. Some people, however, have found a good reason to count them.

For the last two years environmentalists like Lisa Dreves have been keeping tabs on the crows. A decline in the population, she said, could mean something has gone terribly wrong.

“If our crow population starts to come down, when they are so happy living with us in urban areas, then that could mean we have done something really bad,” said Dreves, chair of the Langley Field Naturalists, the group responsible for organizing the count. “Either climate change could be impacting them, or it could be loss of habitat for them to nest in.”

Thousands of birds from across the Lower Mainland roost together in a single area during winter for safety. Despite being large, crows can still get picked off by other birds, like bald eagles. But the crow count isn’t meant to monitor the impacts of their predators, but rather larger elements at play that might impact their ecosystem. The count is just easier to do when the birds are roosting.

Dreves and other volunteers spent four days this year counting the roost.

The birds fly from east Abbotsford, north Langley, eastern parts of Surrey and the U.S. to roost on a small forested area surrounded by farmland, just northwest of the Aldergrove border crossing.

Prior to Aldergrove, the birds roosted in north Langley, Dreves said. Trees cleared to make way for new homes and businesses forced the crows to find a new winter resting place. Whether that disruption had an impact on the size of the roost is unclear because the count was started in Aldergrove just two years ago.

The annual count will illustrate the trend in the crow population. Volunteers estimated there are 53,000 birds roosting on the farmland this year, what they consider to be a healthy number.

So how do they count thousands of constantly moving birds?

“Everybody has a different method,” Dreves said.

Some volunteers count how many crows soar over them each second and multiply that by the time it takes for that swarm to pass. Others on the team are experienced birders who can estimate the size of a large flock from its appearance.

Dreves prefers to count 10 birds to get a visual of a group that size. Then she picks a spot in the sky, sometimes it’s a powerline or a tree, and as the birds fly past that point she uses a clicker to count groups of 10 to get an approximation of the flock.

Getting an accurate count requires at least eight volunteers stationed in four groups around the Aldergrove farmland. The count from each of the four days is then averaged.

The crows typically roost from October to March, and during that time reinforce their own unique social structure.

“They also have a bit of a pecking order, where the crows that are higher up on the hierarchy get to be higher up in the trees, and then the other crows further down the trees get pooped on,” Dreves said, laughing. The juveniles who are coming into maturity also find a mate at that time.

By April the birds leave to nest elsewhere. But they don’t return to the same nest each year. Dreves says each crow family will find a new spot for the female to lay eggs.

“The family takes care of the young and even the last year’s brothers and sisters will also sometimes be around to help take care of their new little siblings.”

The roost has been in the Aldergrove area on unfarmed ALR lands for “many years,” Dreves estimated. But should the property-owner decide to farm that land, the future of the roost is, as Dreves put it, “in the air.”