Pact With Builder Saves Habitat for Gnatcatcher : Development: The agreement will allow construction of 3,000 homes in Carlsbad.

About 500 acres of threatened gnatcatcher habitat in Carlsbad will be preserved by a developer under an agreement that government officials hailed Wednesday as the first of its kind.

The agreement, which clears environmental obstacles to the widening of Rancho Santa Fe Road and construction of 3,000 homes near La Costa, comes as state and federal agencies debate whether to declare the gnatcatcher an endangered species.

“This shows you can have both construction and conservation,” said Bill Daugherty of the Audubon Society’s Buena Vista chapter in Oceanside. “The Endangered Species Act is not the bugaboo that everyone says it is.”

Carlsbad Mayor Bud Lewis said: “We’ve crafted a plan that strikes an effective balance between environmental needs and . . . a strong local economy.”

Under the agreement unveiled Wednesday, Fieldstone Co. will set aside about 500 acres of coastal sage scrub on its 2,300-acre property in southern Carlsbad. The developer will also buy and preserve an additional 200 to 300 acres of sage scrub at an undetermined site elsewhere in the area.

Coastal sage scrub, which grows only in Southern California and Baja California, is the only vegetation in which the small, blue-gray gnatcatcher can survive.

An estimated 48 pairs of the songbirds live on the Fieldstone property, consisting of three separate sites east of El Camino Real in southeastern Carlsbad, according to U.S. Fish and Wildlife officials.

In exchange for setting aside gnatcatcher habitat, over the next 12 to 15 years the developer will be allowed to build a mix of estates, single-family houses and condominiums in the area. About 300 acres of sage scrub will be destroyed during construction.

The agreement also permits a joint city-developer effort to widen Rancho Santa Fe Road, which passes through the southeastern part of the project, from two to six lanes.

After gnatcatchers were found in the area last August, city officials and Fieldstone began negotiating with state and federal officials to prevent delays in development.

About the same time, California officials rejected a proposal to list the bird as endangered, but federal officials in September began proceedings to put the bird on the endangered species list.

In an attempt to head off a federal designation of the gnatcatcher as endangered, Gov. Pete Wilson has been trying to persuade developers to voluntarily set aside sage scrub.

The agreement was negotiated as if the bird were already listed as endangered, said Jeff Opdycke, field supervisor with the U.S. Fish and Wildlife Service.

Glenn Black of the state Department of Fish and Game said the agreement is “totally in sync” with Wilson’s voluntary program.

Jon Atwood, who led the drive to list the gnatcatcher as endangered and who is considered the world’s foremost authority on the bird, was involved in the negotiations and approved of the agreement, according to Dan Silver, who heads the Endangered Habitats League, a coalition of 33 Southern California conservation groups.