The greatest aerial show on Earth, the migration of 1 billion birds on the Pacific Flyway to California, is a full go this week as early winter weather hits Canada.

One of the best predictors of bird migrations and pending winter weather is to track sandhill cranes, both their numbers and routes. A check this week shows the birds are right on schedule with the first arrivals at Woodbridge Ecological Reserve near Lodi, where the Department of Fish and Wildlife is offering weekend tours to see them. In the next three weeks, some 5,000 are expected in this region.

In addition, several hundred sandhill cranes arrived this month at Llano Seco unit of the North Central Valley Wildlife Management Area, reported Dakota Johnson of the Sacramento National Wildlife Refuge. That’s an anomaly for Llano Seco, located 10 miles southwest of Chico, which has a public viewing deck that looks out over a pond and adjoining wetlands.

Pintail ducks, white-fronted geese, shovelers and even some early-arriving snow geese have settled in at the Sacramento National Wildlife Refuge complex in the Sacramento Valley, according to refuge managers. “There’s already a little bit of everything,” Johnson said.

Driving tours are open at the Sacramento unit off I-5 north of Maxwell and Colusa unit off Highway 20.

As the birds migrate south on the flyways and into California, they will look down and see that state wildlife areas and federal wildlife refuges in the Sacramento and San Joaquin Valleys are receiving water to flood wetlands — another benefit from last winter’s big rain and this spring’s snowmelt.

The early delivery of water at refuges and wildlife areas has also pushed wild pheasant out of the low-lying habitat. On the driving tour last week at the Sacramento refuge, field scout and photographer Steve Goodall flushed more than a half dozen pheasant, he said. “They surprise you,” he said with a laugh.

Water, everywhere

There’s another incentive to flood all wetlands available in the Sacramento Valley in the next 10 days. The rice harvest is late this fall — less than 20 percent has been harvested, according to one farmer — pushed back by unseasonal rains in late September. To keep the arriving ducks off the rice fields and limit rice predation as best possible, managers will flood wetlands to provide alternative destinations.

At the same time, severe cold weather hit Canada on Tuesday. In Calgary, the low was 25 degrees with wind and driving snow made it feel like 8, according to the National Weather Service. That will likely propel another wave of birds to launch from “North America’s Duck Factory” and fly south on the flyways, right on schedule.

This year’s fall flight forecast by the U.S. Fish and Wildlife Service tells the story of a healthy duck population across North America. Aerial surveys over breeding areas counted 41.2 million breeding ducks, which is 17 percent above the long-term average over the past 60 years.

Wildlife refuge driving tours Sacramento National Wildlife Refuge: 6-mile driving loop tour with viewing deck, located east of I-5, north of Maxwell Colusa National Wildlife Refuge: 3-mile loop tour with viewing deck, located south of Highway 20, west of Colusa Llano Seco wildlife sanctuary:Viewing platform and trail, located northeast of Willows, southwest of Chico Getting there:$6 or buy federal duck stamp; U.S. Fish and Wildlife Service, 530-934-2801, www.fws.gov/refuge/sacramento Federal Duck Stamp:$25, of which 98 cents of every dollar goes to the purchase of wetlands, provides free access to all national wildlife refuges; available at www.duckstamp.com State Wildlife Areas Woodbridge Ecological Reserve: Sandhill crane tours on weekends, located near Lodi; Woodbridge Ecological Reserve,209-234-3435, info and register for crane tours at www.dfg.ca.gov/delta/cranetour Sandhill Crane Festival: Nov. 1-3, Lodi, 800-581-6150, www.cranefestival.com Cosumnes River Ecological Reserve: Spotting scopes often available for bird watching, located near Highway 99 at Galt;Cosumnes River Preserve, 916-684-2816, www.cosumnes.org Bay Area The Bay Area has 20 wetland marshes that attract migratory shorebirds and waterfowl for winter. The best include: San Francisco: Crissy Field Wetlands North Bay: Napa-Sonoma Marshes Wildlife Area, Bothin Marsh, Giacomini Wetlands, Petaluma Marsh, Richardson Bay, Miller/Knox Regional Shoreline, Whittell Marsh, Grizzly Bay, Grizzly Island Wildlife Area South Bay: Baylands Nature Preserve, Hayward Regional Shoreline, Don Edwards San Francisco Bay National Wildlife Refuge, Foster City Lagoon Coast: Abbotts Lagoon, Bolinas Lagoon, Big Lagoon/Redwood Creek, Pescadero Marsh Natural Preserve Lower Delta: Big Break Regional Shoreline, Frank's Tract State Recreation Area North Richmond Shoreline Festival: Saturday, 11 a.m. to 4 p.m., Point Pinole Regional Shoreline, 520-812-1903, www.northrichmondshoreline.org — Tom Stienstra

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As the fall migrants arrive, there’s one other key factor: The duck season starts Oct. 19. As soon as the first shots are fired, it seems that 99 percent of the ducks immediately fly into the no-hunt zones on the wildlife refuges, often within range of the driving tours.

This year’s procession

Based on projections at the Sacramento National Wildlife Refuge, about 1 million waterfowl have arrived at the Sac NWR complex. Counts from the first aerial surveys in California aren’t due for another week or so, Johnson said.

By the end of October, as the en masse arrival of pintails and snow geese ensues through November, it’s common for the numbers to approach 1.5 million waterfowl. That makes Thanksgiving week the most popular time of the year for the refuge driving tour — a short break off I-5 for holiday travelers — often with clear weather and high bird numbers.

At the height of migration, roughly 3 million waterfowl will spend winter at wetlands in the Central Valley. Roughly 5,000 sandhill cranes are expected at Woodbridge, Staten Island and the Cosumnes River Ecological Reserve.

In the Bay Area, 20 major wetland marshes attract roughly 1.2 million migratory shorebirds and waterfowl for winter. Numbers often peak from Thanksgiving through February.

This time around, it’s Phase 1, Launch Mode, for waterfowl’s biggest aerial show of the year.

Tom Stienstra is The San Francisco Chronicle’s outdoors writer. Email: tstienstra@sfchronicle.com Twitter: @StienstraTom