Unfrequented sandy beaches close to estuaries and coastal embayments had traditionally served as nesting sites for the California least tern. Primarily due to coastal development and intense human recreational use of beaches, the least tern was denied suitable nesting areas and was listed as Endangered with extinction in 1970. A few least tern nesting areas are on beaches (such as, Tijuana Estuary, Naval Amphibious Base-Coronado, Santa Margarita River mouth, Huntington and Venice Beachs). The majority of nesting areas are on manufactured (inadvertently and intentionally) substrates or fills. Since 1970, nesting sites have been recorded from San Francisco Bay to Bahia de San Quintin, Baja California. The nesting range in California has apparently always been widely discontinuous, with the majority of birds nesting in southern California from Santa Barbara County south through San Diego County.

This exclusive fish-eater typically feeds mostly on topsmelt, northern anchovy, and jacksmelt. Feeding is carried out both in the calm waters of narrow estuaries or large bays and for a short distance (i,e., usually within 3 kilometers off beaches in the open ocean. Their hovering and plunging habits are conspicuous.

The California least tern is migratory, usually arriving in its breeding area by the last week of April and departing again in August. Least terns are colonial nesters but do not nest in as dense a concentration as other tern species. Minimum breeding age is 2 years. The nest of the California least tern is a simple scrape or depression in the sand and one to four eggs are laid, usually two. There is one breeding season, from May through August and only one brood is raised. However, the birds will renest if eggs or chicks are lost. Re-nesting attempts after initial failures, and 2-year-old birds nesting for the first time, often occur from mid-June to early August. Parents continue to feed fish to their young even after they are strong fliers.

The least tern is vulnerable to a long list of predators, some of which are very abundant in urban environments, such as feral cats and dogs, crows, and American kestrel. To protect least tern nest sites from disturbance, ground access is typically controlled by fences or other obstructions. Nest site management requires these three elements to improve least tern reproductive success: protection from disturbance, management of predators, and surface preparation where weeds or excessive plant growth is a problem.

At the time of listing as Endangered, the least tern breeding population was estimated to be about 600 pairs. The Statewide breeding population has increased considerably in just the last five years and has exceeded 4,500 pairs since 2000.

When the Bolsa Chica nesting islands were created in 1978, the least tern began nesting on the north island. When elegant terns, then black skimmers, began nesting on the north island, the least tern was crowded off of it and began nesting on the south island. In recent years, least tern nesting success at Bolsa Chica has been very poor, largely due to predators and the relatively small suitable area for nesting.