Bree Jacks says it’s tough to have a girls’ night out in San Jose. There are just too many guys, drawn like ducks to bread crumbs, “googling” any group of females for a potential hookup.

“I think it’s pretty good for the girls,” said Elizabeth Harris, a recent transplant from Los Angeles who finds San Jose’s legions of eager men “refreshing.” She was the only woman sitting at a bar lined with men at downtown’s Mission Ale House — aka: the Mission Male House. “You can be more picky. They have to try harder. They all try to one-up each other.”

But while Mountain View, Sunnyvale, Milpitas and Santa Clara are even more heavily male than San Jose, new census data has unearthed the surprise existence of verifiable female turf in Silicon Valley. Some of the most affluent communities, including Los Altos, Los Gatos, Cupertino and Saratoga, are predominantly female, even among young adults.

In Los Altos, Mayor Megan Satterlee couldn’t explain the mystery of why there are only about 85 men for every 100 women among adults aged 20 to 44, and she wondered how those newly released numbers from the U.S. Census Bureau could be true.

Nevertheless, a two-block stretch of downtown Los Altos shouts out the proof: Two dozen hair salons, nail parlors and clothing boutiques dominate the landscape with nary a sports bar in sight. “There used to be a men’s store,” Satterlee said. “It used to exist but it just kind of died out at some point.”

The numbers rang true to three 18-year-old dudes — Justin Carella, Mitch Parsons, and Brian Isbell — happily hanging out recently in Los Altos. In neighboring Mountain View, where Isbell lives, the Census Bureau says there are about 125 men for every 100 women.

“Why,” Isbell said with a big grin on his face, “do you think I’m here?”

To be young and single is to know the monikers for the imbalanced social life in the capital of Silicon Valley — Man Jose and the Guy Glut.

Guess what? With the workforce of some of Silicon Valley’s largest companies more than two-thirds male in recent years, the Valley’s gender imbalance has grown. San Jose has one of the most heavily male populations among the nation’s big cities, according to the new 2005-07 data.

The mismatch is particularly acute — or favorable, depending on your point of view — for younger adults. Among people between 20 and 45 years old, 54 percent of San Jose’s population lifts the dreaded toilet seat, meaning there are about 117 men for every 100 women. That’s up from about 111 men for every 100 women since 2000.

Silicon Valley is far from being the most male place in the country, a distinction that belongs to cities with prisons and military bases. But, at 51.3 percent male, San Jose is even more male than San Francisco, despite the clustering of gay men in the Castro district.

There’s no mystery about why much of Silicon Valley is so male-dominated, said Andrew Hacker, the author of “Mismatch: The Growing Gulf Between Men and Women.” He says just look at the tech jobs in Silicon Valley and the graduating classes of engineering schools, which remain overwhelmingly male.

“Hey, it’s a bit like the mining camps” of the California Gold Rush, where young men come to make their fortune, Hacker said of the Valley.

Indeed, migration of men from other states, rather than immigration from abroad, is what created the imbalance.

Of the young U.S.-born adults who moved to Santa Clara County from other states, 56 percent are male, according to Hans Johnson, a demographer for the Public Policy Institute of California.

“It is the nature of the labor market that tech and computer science workers are more likely to be male,” Johnson said.

More than two-thirds of the workforce is male at several big Silicon Valley companies, including Cisco Systems, Intel and Advanced Micro Devices, according to 2005 data obtained by the Bay Area News Group through a federal Freedom of Information request.

Other companies, including Google and Yahoo, refused to comply with the request, claiming the information as a trade secret or a worker privacy violation. The gender imbalance decreases at older ages, as men — apparently the less fit of the two sexes — die off.

But for younger men like Rigo Pantoja, a 25-year-old bartender who sees plenty of male-female interactions at the downtown club where he works, San Jose’s gender imbalance is a problem.

“It’s the attitude girls have because they can be picky, so they can blow off any dude they want. And I think guys have to throw money at girls now to get more attention,” he said. “Pretty much the girls I know go to Campbell or Santana Row, because that’s where all the guys with money go.”

Meanwhile, others offer that gender imbalance makes the night life business in the Valley tougher.

“It is hard to get the women in here,” said bartender Tricia Thoren at the Mission Ale House, which has held Ladies Nights and other events to battle the boys club reputation. The numbers of men downtown can be so large, Thoren said, that many women can feel intimidated.

“One or two women won’t come in by themselves because they’ll look inside and see all these guys, and it’s like, oh my God,” said Wolfe Cevorov, who works in a downtown club. “A lot of my girlfriends are going to other places, like Palo Alto, San Francisco, because it’s gotten so big, these crowds of men.”

The gender imbalance may be causing relationship problems at one end of Silicon Valley, but relationship problems may be causing the gender imbalance at the other. In Los Altos, for example, census data shows there are more than twice as many divorced or separated women as men.

“Those are family towns,” said Johnson, the PPIC demographer. “It might be likely that when people get divorced, the wife ends up with primary custody of the kids and the house, and the man ends up moving out.”

Maybe to Mountain View.