SAN DIEGO — Weeks into a capricious fire season that has already burned parts of Catalina Island, Los Angeles and Lake Tahoe, swaths of California's flammable national forests are protected some days by nothing more than luck.

On any given day, about 40 out of 271 U.S. Forest Service engines in the state remain in firehouses rather than on routine patrol, idled by a shortage of supervisors as the combined effects of sustained drought, last winter's freeze and a searing heat wave send fire danger levels into "extreme" territory.

The cuts are one effect of an exodus of highly trained mid- and upper-level firefighters from the career ranks of the service, leaving 13 percent of the agency's 3,600 full-time positions in the state vacant.

Some firefighters fear those gaps could strain the ability of federal fire crews to respond quickly to fires, leading to more out-of-control blazes in what promises to be a tough fire season.

"When you start leaving holes in your organization so that on a given high-danger day you can't provide coverage you've set yourself up for trouble," said John Marker, a retired former Forest Service district ranger on the Sequoia National Forest.

Nationally, fire planners from all five federal agencies that handle firefighting are dealing with the departure of a generation of top managers hired during a firefighting expansion in the late 1970s, leaving behind too few career firefighters qualified to run engines, oversee forests or command large fire operations.

As forests from the Mexico border to Canada reassign engine crews, top-level teams working for other agencies are simply hiring recent retirees. Of 50 people working on one Nevada-based National Park Service squad, 10 are due to retire in the next two years, and a handful have come out of retirement as emergency hires this season.

"We haven't been able to fill out teams so we keep bringing back the old warhorses," said Paul Broyles, who heads the Nevada team. "There's a gap in the pipeline because we didn't get enough people in the pipeline in the 1980s, and there are fewer and fewer people who want to stay long enough to rise to the top."

California has been hit harder than other states, because the high cost of living has deterred recruits from moving here, while state and local agencies are replacing baby boomers as they hit age 50 and siphoning federal managers with higher pay and better benefits.

Forest Service officials have filled nearly 800 positions since last October, but are still short about 470 people.