Tour the Tijuana Estuary National Estuarine Research Reserve for a look at some of the 400+ species of birds that visit or nest here.

Four square miles of marshes, tidal creeks, and sage-and chaparral-covered hillsides in and around the Tijuana River Estuary enjoy federal protection under the tongue-twisting title "Tijuana River National Estuarine Research Reserve." The area includes Border Field State Park, fronting the international border, and undeveloped tracts of land adjacent to the communities of Imperial Beach and San Ysidro.

Unlike the remnant bits of salt marsh surviving along other parts of the Southern California coastline, the broad Tijuana Estuary has survived, relatively unscathed, the advance of urbanization and its attendant destruction of wetland habitat. It is quite startling to gaze upon a vast, flat variegated carpet of gray and green, scored by a network of curving tidal creeks and brackish ponds, and realize that you are also looking deeply into California's past.

It's best to begin your exploration of the Tijuana River estuary and marsh with a stop at the reserve's visitors center on Caspian Way near Third Street in Imperial Beach (open 10 a.m. to 5 p.m. daily). As you approach the entrance, your footsteps take you across a stylized map of the Tijuana River drainage, which includes parts of the Laguna Mountains and a large section of Baja California. In addition to interactive exhibits inside the visitors center, a demonstration garden outside shows off various species of the typical yet fast-disappearing coastal sage-scrub vegetation that once blanketed San Diego's coastal mesas and hills.

This time of year, the big attraction in and around the marsh is birds. More than 400 species have been logged here so far, which is no surprise since the estuary is a key stopover on the Pacific Flyway -- the equivalent of Interstate 5 for bird-migration traffic. The reserve's best-known endangered inhabitants are the light-footed clapper rail and the California least tern. Visiting birds include ospreys, golden eagles, and peregrine falcons.

From the visitors' center, you can follow wide, marked trails west or south around the perimeter of the tidal basin -- the area subject to the incursions of both seawater and freshwater. You can also follow a path south from the end of Fifth Street, which will take you farther south and west toward the Tijuana River. Or you can start at the south terminus of Seacoast Drive for a walk along the beach (but not the dunes alongside, which are closed to protect nesting terns).

Call the visitors center, 619-575-3613, for more information about the reserve or to learn about and make reservations for guided nature hikes and special events.