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Squirrels look for food in a parking lot along Thornton Ave. on Wednesday Aug. 25, 2010 in Newark, Calif. (Aric Crabb/Staff)

Hundreds of ground squirrels lounge and frolic in the grass at Martin Luther King Jr. Regional Shoreline in Oakland these days -- more than park managers have seen in years.

West of Martinez, gray splotches of crushed field mice dot a mile-long stretch of Highway 4, after a large and unusual mouse migration across the freeway this month.

In Danville, Todd Bettencourt says his cat Rose has brought home baby field mice, also called voles, several times this summer. "I never seen so many of them," said Bettencourt, who nursed two of the voles to health before releasing them near a nearby creek.

Rodent populations are booming in many Northern California areas after the first wet winter and spring in several years led to abundant grasses and plants that feed the rodents and hide them from predators.

"It's a banner year for rodents," said Doug Bell, wildlife biologist with the East Bay Regional Park District, operator of 65 parks and wilderness areas in Contra Costa and Alameda counties. "Nature goes through these cycles. It just seems that California goes through these booms and busts."

Not only did extended rains raise many wild plants, but cooler summer weather has kept that vegetation green longer.

Doing especially well are ground squirrels and voles, a short-tailed field mouse bigger than a house mouse but smaller than a rat.

The surge in rodent numbers is a boon to hawks, owls, eagles, rattlesnakes, coyotes and other wildlife species that rely on ground squirrels and voles for food.

"I'd say it's a banner year for about everything, but especially for voles and rabbits," said Kevin Pendergraft, an Alamo resident who operates the California Exterminating Service.

Regional park officials said they have taken no measures to kill the rodents because they haven't threatened to undermine structures or overrun picnic areas.

But complaints have increased to county agricultural officials and commercial pest controllers.

"The number of calls we're getting are up," said Vince Guise, the Contra Costa County agricultural commissioner.

His workers are busy poisoning ground squirrels to protect levees and roads from being undermined by burrows, he said. His department also is giving advice to more homeowners about control methods.

Last week, the state Department of Fish and Game cautioned homeowners and property owners statewide to use caution when trying to poison rodents because of the secondary risk to owls, hawks, coyotes or other predators that may eat the poisoned creatures.

"Our message is that if people are going to use approved products, they should read the labels carefully on how and where to apply them," said Stella McMillin, a state fish and game environmental scientist.

Since 1994, the Department of Fish and Game has confirmed 136 cases in which rodenticides -- which disrupt animal blood clotting -- have sickened or killed wildlife, including mountain lions, foxes, black bears, raccoons, eagles, owls and vultures.

In a boom year like this one, voles chow down on alfalfa and other crops, strip tree bark and eat plants in gardens.

Vole populations can explode in a good year. One female gives birth several times a year, and the newborns can start reproducing in about seven weeks.

Spencer Adams, of Rodeo, spotted signs of a mass vole migration this month as he drove westbound on Highway 4 near Martinez. He saw hundreds of brownish gray splotches in a mile stretch of freeway west of Alhambra Avenue.

Adams figured it was a mouse migration because he had heard of something similar years ago on Cummings Skyway near Crockett. Some friends ridiculed him, but Bay Area News Group pets and wildlife columnist Gary Bogue inspected the flattened remains and confirmed they were voles.

Biologists speculated that the voles crossed the freeway in search of greener pastures after they gobbled up their food sources or saw them dry up and wither.

"Humans can cultivate food and move it long distances, but the rodents surviving in the wild need to move when their food source is used up," said Jerry Roe, a Martinez wildlife biologist.

Vole populations typically drop sharply when food sources aren't abundant.

"Survival rates of voles aren't very high, and they live for a relatively short time of 2 to 16 months," said Roger Baldwin, a University of California integrated pest management adviser in Parlier near Fresno. "We get these cyclic booms, but things have a way of evening out."

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff.