Today through Jan. 5, bird enthusiasts across North America and beyond will participate in the National Audubon Society’s 112th annual Christmas bird count.

Considered the world’s longest-running citizen science survey, last year’s count involved 62,624 volunteers who spotted more than 61 million birds throughout the United States, Canada and some of Latin America and the Pacific Islands, according to the society.

In Maine, the one-day counts will start Saturday, when teams of avid birders from Greater Portland to Old Town to Jonesport will fan out across designated study “circles” that each measure 15 miles in diameter or 177 square miles. There are 28 study circles in the state, each encompassing a variety of woods, fields, rivers, beaches and backyards.

“We spend all day in the woods,” said Marie Jordan, 73, a retired teacher who lives in South Portland. “It’s a fun day in the field, contributing to citizen science. It’s also a challenge, year to year, to see what you’ll find.”

A 25-year veteran of the annual survey, Jordan will take part in four counts over the three-week period: Greater Portland on Saturday, York County on Monday, Biddeford-Kennebunkport on Dec. 31 and Freeport-Brunswick on Jan. 1. She heads the Biddeford-Kennebunkport count.

The Christmas count is one of several bird surveys conducted each year. It helps scientists track the health of bird populations and their habitats, including changes in climate and food sources. The data also informs the annual State of the Birds Report, issued by the U.S Department of the Interior.

“It’s a globally recognized example of crowd science,” said Gary Langham, Audubon’s chief scientist, noting that the Christmas count also “does good things for families, communities and the conservation movement.”

During the Christmas count, surveys of individual study circles are scheduled in advance and take place regardless of cold temperatures or foul weather, said Bill Hancock of Gray, a former Maine Audubon employee who heads the Greater Portland count.

This year’s recent warm temperatures and lack of snow cover may produce lower counts, Hancock said. Birds have yet to be cut off from natural food sources, so many haven’t been visiting bird feeders and remain dispersed in the wild.

Still, volunteers will be diligent, Hancock said. Study circles in more populated areas, such as Greater Portland, may be surveyed by more than a dozen teams, each with two or more members. Study circles in more rural areas may be surveyed by a handful of volunteers.

The Greater Portland survey typically attracts more than 40 volunteers and spots more than 100 different species each year, Hancock said.

The center of the Greater Portland study circle is at Route 77 on the South Portland-Cape Elizabeth line, with the outer reaches ranging from Peaks Island to Prout’s Neck in Scarborough to Westbrook to Falmouth’s town landing.

Last year’s Greater Portland survey recorded 110 species, including 744 black-capped chickadees (the state bird), 236 common loons (an unusually high count for the beloved waterfowl) and one American woodcock (considered an unusual species).

Teams follow assigned routes and often cover the same area year after year. Some volunteers start at 2 a.m. to catch the nocturnal activity of owls. They count every bird they see, which can be difficult if volunteers roust a flock of crows. They gaze through binoculars until the light fades, then compile their results in the evening with other volunteers.

Despite the hard work ahead, Linda Woodard of Kennebunkport is looking forward to Saturday’s count in Greater Portland. She’s been tallying birds along the Presumpscot River corridor for about 20 years, sometimes wearing snowshoes to get the job done.

One bird sighting several years ago made a particular impression on Woodard, who is director of the Scarborough Marsh Audubon Center.

She was walking across the Riverside Golf Course in Portland when she slipped on a patch of ice and landed flat on her back.

Uninjured but dismayed, she gazed up at the sky, wondering why she does the count each year.

“Suddenly, this red-tailed hawk circled over my head and called out several times,” Woodard said. “It was an affirmation that yes, this is why I do this. It was beautiful.”