When the crows go crazy in the treetops, caw-cawing and flapping their black wings, Evan Houston scans the bare winter branches. Often, the crows are responding to a raptor nearby.

The Christmas Bird Count, sponsored by the Audubon Society, is a 100-year-old holiday tradition that is both science and hobby: a light-footed stroll to collect a citywide bird census.

It's been going on nationally since 1900, originally a protest to the traditional Christmas bird hunt popular with feather-hunters.

About 150 volunteers donned boots and binoculars in Seattle for the 2011 count Saturday morning. They met at 8 a.m., just as the sky was turning from slate gray to pink, and padded through the crisp dawn across marshes, along shorelines, through neighborhoods and parks.

At least one expert birder led each group. Treetops and underbrush came alive as the experts interpreted each chirp and warble. Brown creepers circled tree trunks to find bugs in the bark, seeming to crawl along on their white-feathered bellies. Tiny redheaded Anna's hummingbirds — an increasingly common species in Seattle — buzzed from perch to perch. Flicker woodpeckers sat in tall trees, while in Yesler Cove on Union Bay near the University of Washington, a great blue heron stood stoically, watching the water for fish.

Birders are unhurried and keenly observant. They perk up at the sound of birdsong that would go unnoticed by an ordinary hiker, train their binoculars on the brush to find the twitch of a sparrow preening in the underbrush or a chickadee hanging upside down to hunt for food.

"I find birds are a great focal point for enjoying nature," said Houston, a community-college biology teacher and master birder who led a group through the Union Bay Natural Area, also known as the Montlake fill, near the University of Washington.

Volunteer counter Maureen Brown said she walks to the fill often from her Montlake home, looking for new birds and training her eyes to watch for flashes of red that distinguish male from female, or the wing shape that distinguishes heron from eagle in flight.

"I like it because it forces you to slow down a little bit and really pay attention to the detail," she said.

The young-owl population has been high in 2011, said Audubon Society science manager Adam Sedgley. That means the possibility of a snowy owl in this year's count as they winter farther south than usual. Several have been spotted nearby in 2011. Other things counters were watching for: observations of declining species such as the California quail and ring-necked pheasants.

Barred owls are on the increase, as are Anna's hummingbirds and Brant, a type of goose.

"It's not controlled science or anything. We're still doing citizen science," said Colene McKee, who organized this year's count. "Audubon kind of wants it to be a community activity in a way. It's a way to bring in new birders."

Emily Heffter: 206-464-8246 or eheffter@seattletimes.com. On Twitter @EmilyHeffter.