In Northern California and around the world, thousands of bird enthusiasts are bundling up, grabbing binoculars and heading outdoors to count birds at the crack of dawn.
The Audubon Society's 114th Christmas bird count is midway through its annual three-week run, but early reports indicate that some birds are shunning California, likely because of drought conditions and unseasonably warm weather.
Of all the factors that can affect bird numbers, "water is certainly the most important," said Garrison Frost, director of marketing and communications at Audubon's San Francisco offices.
Volunteer "citizen scientists" in Oakland, the Central Valley, and Lake and Mendocino counties have reported that the numbers of waterfowl, shore birds and some others appeared to be down so far during this year's Christmas bird counts, held Dec. 14 through Jan. 5.
Those and other birds like to feast on the plants, insects and other creatures that are nurtured by water, Frost said. If there's a shortage of food, they won't stop by, he said.
In Mendocino County, there was no shortage of bird species — 129, about average — during the Peregrine Audubon Society's Ukiah-area count, held Dec. 14, said Bob Keiffer, a board member and superintendent of the University of California's Hopland research and extension center.
But bird counters reported seeing fewer waterfowl, including scaup, a diving duck that normally is plentiful, he said.
Keiffer said unseasonably mild weather could be a factor. There haven't been enough heavy storms to push all the birds south from their summer homes, he said.
"They only go as far as they have to," he said.
Darlene Hecomovich, who organizes Lake County's Redbud Audubon Society Christmas bird counts, agrees. "As long as there's food where they are, they don't want to move," she said.
Hecomovich said her group's Dec. 14 count turned up 36,657 birds, a decline from the preceding 10-year average of 48,530. But it's up from last year's 24,114. The 10-year average includes a low of 23,313 in 2008 and a high of 135,312 in 2004, she said.
Another factor that affects bird counts is the number of people who participate, she said. There tend to be more volunteers when the weather is nice, Hecomovich said.
Frost said what causes birds to be plentiful some years and scarce in others is uncertain.
"There's so much about migration that is a mystery to us, really," he said.
The Santa Rosa-based Madrone chapter of the Audubon Society could not be reached about its Dec. 15 bird count, but birds in the Petaluma area appear to be abundant, according to Mary Edith Moore of the Petaluma Wetlands Alliance.
The group does its own bird counts on a regular basis. On Dec. 11, participants counted about 6,000 birds and 62 species at Shollenberger Park, Moore said.
"I think it's pretty good," she said.
There's still time to take part in a Christmas bird count; the groups welcome new volunteers, whom they train on the job, as well as experienced birders.
The Sonoma Valley Audubon chapter is holding its Christmas bird count on Dec. 27, according to its website. Counts are scheduled from dawn to dusk, but volunteers needn't stay the entire time.
The most dedicated of the birders often are out well before dawn. During the Ukiah-area count, the group's president, Chuck Vaughn, and two others ventured out in the dark to survey owls.