LAWRENCE — Sitting in their station wagon atop a hill in the Immaculate Conception Cemetery on a cold, windy afternoon, Dana Duxbury-Fox and her husband, Bob Fox, scan the sky with binoculars.
Suddenly, hundreds of crows begin to appear, flying in from different directions to their nightly roosting sites.
“Oh my God, look at them!” Duxbury-Fox said.
“They come almost like rivers, flowing down in the direction of where they’re going for their night roost,” Fox said.
“Notice the time,” Fox said, pointing to his wristwatch. “It’s 4:30. Sunset is at 4:59 today. Half an hour before [sunset] almost to the minute, they start. It’s uncanny . . . ”
When all arrive, Fox estimates there could be 15,000 of the big black birds, comprising one of the largest winter crow flocks in New England. He said the crows roost from November to March, but January and February have the highest numbers. Other large flocks in the region have been reported from Springfield and Hartford, Fox said.
The North Andover couple are experienced birders. They have traveled all over the world and seen more than 6,000 species of birds, Duxbury-Fox said. But, the Lawrence winter crow flock, which they’ve been following for four years, is clearly one of their favorites. Duxbury-Fox said that during the winter roosting season, she and her husband come out every few nights to follow the crows and report on their movements.
“We have taken folks there to see it and they leave amazed,” Duxbury-Fox said.
Crows were first reported roosting in Lawrence in the winter of 1989, she said. In recent years, the Lawrence winter roost was in trees along the Merrimack River near the New Balance factory store.
“Suddenly, at the end of December they were staging on the ice west of Route 28, and roosting to the east and west of there,” Duxbury-Fox said. “On the 9th of January we followed them to their new staging and roost site at the Immaculate Conception Cemetery in Lawrence.”
Staging refers to the crows gathering around sunset in places such as parking lots, on the surface of the ice on the Merrimack River, and more recently, on the ground at the cemetery, prior to flying to trees to roost for the night. Duxbury-Fox said the birds are usually in their roosts by about half an hour after sunset. That night, the crows settled in a residential neighborhood in nearby Methuen.
It’s been speculated, said Duxbury-Fox, that one of the reasons some crows roost together in winter is because there’s safety in numbers.
“Their natural predator is the great horned owl,” Duxbury-Fox said. “That bird will go in and just pick them off in the night. And it’s a powerful bird.”
“Crows, in reverse, will mob around an enemy like that,” Fox said. “And so, if he sits down in a tree, they’ll be sort of dive-bombing, trying to scare him out and scare him away.”
Wayne Petersen, director of Mass Audubon’s Massachusetts Important Bird Area Program, said in an e-mail that other reasons why many crows roost together in winter might include crowding together to keep warm, and possibly finding mates.
“Roosts are often comprised of many immature birds,” Petersen said. “A large crow roost for some crows is essentially like a giant seasonal dating bar . . . ”
Petersen said immature crows may learn where to find food by following more experienced crows when they fly off in the morning.
“Research has shown that, in some areas, crows will travel as far as 80 miles a day on a foraging trip,” Petersen said. But he suspects the Lawrence flock forages closer to home.
Petersen said crows are omnivorous, meaning that, like people, they feed on a variety of different things.
Tom French, assistant director of MassWildlife’s Natural Heritage & Endangered Species Program, said in an e-mail that he and Craig Gibson, an avid bird-watcher and photographer from Winchester, collected a large number of regurgitated food pellets — the nondigestible parts of what they eat — from under the Lawrence crow roost in December. French said about 90 percent of the crows’ food appeared to have been berries from Oriental bittersweet, an exotic invasive vine.
French said other food items included an acorn, corn, mice, a snail, a beetle, and a wrapper from a McDonald’s ketchup package.
Duxbury-Fox said the Lawrence flock is made up of permanent residents that nest within 50 miles of Lawrence, and migrants from the north, Canada and the northern New England states. It consists of two species: American crows and fish crows.
Foxsaid fish crows are smaller than the more abundant American crows and have a different call – a short, nasally “uh” or “uh-uh” sound – as opposed to the familiar “caw-caw-caw” of the American crow.
Foxsaid that while American crows are widespread across the country, fish crows are primarily a Southern species that has moved into Massachusetts and makes up only about 1 percent of the Lawrence flock.
In the spring, the crows depart from their winter roosts and typically return to their point of origin, where some breed, Petersen said.
French said people sometimes complain the crows have created a nuisance because of their noise or droppings, but there are no appreciable human health concerns associated with crows or crow roosts.
Gibson said local residents are usually fascinated by the large number of raucous crows.
“A big part of the interest is the mystery of this amazing phenomenon,” Gibson said, “and how it has continued for decades and perhaps centuries.”Don Lyman can be reached at email@example.com.