OCEANSIDE —— With nesting season officially over, work has finally begun on one of the most extensive construction areas for the Sprinter light-rail line.

"The (least Bell's) vireos have gone south for the winter and the gnatcatchers are no longer trying to nest," said Erich Lathers, president of BRG Consultants in San Diego.

All summer, the consulting firm has kept tabs on a half-dozen endangered vireo songbirds and about 10 California gnatcatchers, which are listed as threatened under the federal Endangered Species Act.

The birds, combined with a very wet winter in 2004, have largely kept construction crews from moving much dirt in Oceanside from College Boulevard west to El Camino Real.

But the federally recognized nesting season for the protected birds ended Sept. 15. And rain has not yet made the ground too spongy for heavy equipment.

So, it's time to get some serious earthmoving done.

Ray Hughes, project manager for the Sprinter construction, said Monday that crews had begun driving steel piles deep into the ground in preparation for one of the largest earthmoving efforts necessary to build the 22-mile, light-rail line from Oceanside to Escondido.

"It will be about 100,000 cubic yards of fill we will have to haul in," Hughes said.

In Oceanside, the railroad tracks run alongside Loma Alta Creek. In fact, the existing freight line that the Sprinter will replace is technically inside Loma Alta's flood plain. In order to keep the $385 million Sprinter line high and dry as it carries passengers between Oceanside and Escondido, the rail must be rebuilt atop a dirt berm. That berm will be up to 8 feet high in some parts of Oceanside.

The federally mandated window of work for areas with protected species present ends Feb. 15. Hughes said a dry winter, or at least one with less rain than last year, should allow crews to finish before that window closes.

"Our goal is to get it done by February," Hughes said. "If it doesn't rain, we should be able to finish it in five or six months."

Meanwhile, work throughout the rest of the Sprinter corridor, which roughly parallels Highway 78, has made much progress this summer. Hughes said crews have laid nearly six miles of rail, finishing a 2,600-foot section from Temple Heights Drive west toward College Boulevard.

Oceanside will see a spate of Sprinter-related work this fall, with major railroad crossings at College Boulevard and El Camino Real scheduled for replacement.

There is always a slight chance that construction efforts in the area may not have to stop next February. If biologists hired to monitor the project do not detect endangered or threatened birds in the area, Lathers said construction can continue during nesting season. He noted that, in addition to federal and state endangered species laws, work along the Sprinter corridor is also subject to the Federal Migratory Bird Act, which specifies that nesting seasons for migratory birds not be interrupted.

"The gnatcatchers pretty much stay in the same place for their whole life, but, because they exist in both California and Mexico, they are considered migratory," Lathers said.

Contact staff writer Paul Sisson at (760) 901-4087 or psisson@nctimes.com.