SAN JOSE -- It was a day truly for the birds.

Thousands of volunteers Saturday headed into the wilds around San Jose and other communities nationwide to tally the numbers of California towhee, acorn woodpeckers and scores of other fowl in what has been billed as the longest-running citizen science program in the world.

Organized by the National Audubon Society, this year's 113th annual Christmas Bird Count is intended to provide scientists with an annual census of bird populations to help assess the relative health of different species and guide conservation efforts. Despite the mud, the four hearty souls who hiked around San Jose's rain-drenched Santa Teresa County Park with a photographer and reporter weren't complaining.

"I like the outdoors," said 29-year-old Justin Fairchild, who said he recently graduated from Chico State University with a degree in environmental science and who jotted down on a clipboard how many of each bird type they spotted. "It's nice to go on a hike."

The count was first conducted on Dec. 25, 1900, as an alternative to what had been a Christmas bird-shooting tradition among hunting enthusiasts. That year, it drew 27 volunteers in 25 locations across Canada and the United States. This year, the event -- which runs from Dec. 14 through Jan. 5 -- is expected to attract about 63,000 bird spotters in more than 2,200 areas.

California has an especially sizable pool of participants, with 5,787 marching nearly 7,400 miles in last year's count, according to the Audubon Society. When the volunteers spot something unusual, it often gets mentioned in the organization's annual summary of the data.

Despite laboring from 7 a.m. to 4 p.m., the four volunteers at Santa Teresa County Park couldn't claim to spot every bird that was there, and at times it was hard to determine if some of the ones they saw may have been counted earlier in the day.

"It's definitely not precise," said Jim Thomas, a 68-year-old retired software engineer from San Jose, who was in the group with his 47-year-old son, Jamey Thomas, a portrait photographer. But the elder Thomas, who had participated in several previous counts, said the volunteers always work hard to come up with the best tally they can, adding that the results from year to year have been relatively consistent.

Besides spotting deer tracks and signs of feral pigs, the four volunteers also came across a pack of coyotes, whose howls echoed eerily through the hills. But birds were the stars of the show and they were evident in glorious diversity. Among the 63 species sighted were red-tailed, red-shouldered and Cooper's hawks, kites, kestrels, a rare merlin falcon, ravens, robins, bufflehead ducks, scrub jays, finches and rufous-crowned sparrows.

At one point, Jim Thomas used his smartphone to play the song of a California thrasher in hopes of luring the bird into the open. When one did, he exulted, "that's it. There it is."

And when the volunteers spotted what they said was an intergrade northern flicker, displaying characteristics of both the yellow-shafted and red-shafted versions of the bird, they were just as excited.

"Ooh, that's cool," said 28-year-old Erika Taketa, who has a bachelor's degree in wildlife biology and is studying for her masters in statistics at San Jose State. "A definite intergrade."

Contact Steve Johnson at sjohnson@mercurynews.com or 408-920-5043. Follow him at Twitter.com/steveatmercnews