Crushed oyster shells and curved roofing tiles are being spread this week to protect the nests of California least terns, a rare migratory bird that breeds in the sand near a railroad bridge under construction across the Batiquitos Lagoon in Carlsbad. The California least tern is a white-and-black b ird less than 10 inches long that breeds only in a few areas along the bays and lagoons of California and Mexico. Much of its coastal habitat has been lost to development, and as a result the bird is on the federal list of subspecies in danger of extinction. The oyster shells and tiles will help camouflage the birds’ nests and protect the chicks and eggs from predators, the project managers said. The least tern typically lays its clutch of two or three eggs on the sand in the spring. When the chicks are grown, the birds fly south to spend the winter in Mexico. “This is the fifth most productive least tern nesting site in the state,” said Gabriel Penaflor, reserve manager for the California Department of Fish and Wildlife. Batiquitos Lagoon gets 400 to 600 least tern nests and produces 50 to 100 fledglings per year, Penaflor said. On Tuesday, workers were scattering the shells on a flat sandy area between the railroad tracks and Carlsbad Boulevard, also known as old Highway 101 or the Pacific Coast Highway. The 4-acre site near the highway also was being reinforced with sand from the nearby bridge construction to repair erosion that occurred in recent years. It’s one of several nesting sites created around the lagoon about 1996 as part of an environmental mitigation project that excavated sediment from the lagoon and opened it to tidal flushing. Before that, Batiquitos was essentially a large mud flat cut off from the ocean for most of the year by a rocky berm that built up naturally at the beach. “This is critical habitat for many species of birds, especially the least tern,” said Tim Pesce, a senior environmental planner for the San Diego Association of Governments. Snowy plovers and Ridgway’s rails also are endangered native birds found along the lagoon, but the tern is the species of concern for the bridge project. Crushed oyster shells were spread Tuesday on a nesting site for the endangered California least tern (at lower left) along the Batiquitos Lagoon in Carlsbad. (K.C. Alfred / The San Diego Union-Tribune) The bridge under construction will carry two sets of tracks. It will replace an old single-track wooden trestle as part of SANDAG’s ongoing efforts to add a second set of rails and improve service on the rail corridor between downtown San Diego and the Orange County border. More than three-quarters of the 60-mile route has been double-tracked so far. The toughest and most expensive segments, such as those crossing the coastal lagoons, were left for last. Each project includes an element of environmental mitigation. Construction of the bridge and its footings in the lagoon is expected to produce 50,000 to 60,000 cubic yards of beach-quality sand, Pesce said. About 8,000 cubic yards of that has been placed on the nesting site. The rest will be spread on nearby beaches such as Ponto, starting in January and continuing off and on through 2028 until construction is finished. The bridge replacement also will improve water flow in and out of the lagoon. The old trestle, built about 1940, is supported by sets of wooden posts set 14 feet apart in the water. The new steel-reinforced concrete bridge is about 50 feet longer and is supported by just five sets of posts 56 feet apart between its two abutments, giving it much less of a footprint in the water. “The old railroad bridge constricted the tide,” said Penaflor, the reserve manager. “That’s why we got such a large sand shoal here at the bridge,” he said. “Hopefully, with the new bridge we will have to dredge less, and that will benefit the whole lagoon.” Oxygenated water from the ocean flowing farther into the lagoon will help support all types of wildlife and keep the lagoon healthier, he said. The oyster shells are about the same color as the bird eggs, so they help to camouflage the nests, which are little more than a scrape in the sand. Along with the oyster shells, the crews also will spread about 75 curved clay Spanish roofing tiles around the nesting sites. Carlos Flores, left, Jose Clark, and Ernesto Sierra spread crushed oyster shells to restore the endangered California least tern nesting site near the Batiquitos Lagoon double track project on December 9, 2025 in Carlsbad, CA. (K.C. Alfred / The San Diego Union-Tribune) “It makes a spot for chicks to basically run and hide under,” Pesce said. Coyotes, possums, raccoons and even hawks and owls are among the predators that prey on the baby birds. The Fish and Wildlife Department monitors the sites during nesting season and employs a service to trap and remove the predators. Some are euthanized and others, such as the owls and hawks, are taken to Northern California and released. Vegetation covers only about 10% of the nesting site and is limited to a few native plants such as saltgrass, beach suncup and evening primrose, Penaflor said. The rest is bare sandy soil. Plants that grow too prolifically are removed to preserve the best nesting sites. “The birds like low-lying, flat plants,” he said. “We have to make sure there’s nothing too tall.” In May 2024, SANDAG officials revised their estimate of construction costs for the bridge upward to $165.6 million, an increase of $42.5 million from previous estimates. Construction is prohibited during the least tern nesting season from April 15 to July 31. The bridge and nesting sites are in the Batiquitos Lagoon Ecological Reserve, which includes 544 acres of disappearing coastal habitats such as intertidal mud flats, salt marshes, transitional zones and willow riparian forest. The Batiquitos Lagoon Foundation maintains a nature center and hiking trails on the northern shore just east of Interstate 5. ...read more read less