The California least tern was doing just fine up until the 1800s. It could be found nesting in big colonies along the California coast from San Francisco all the way down to Mexico. However, during the late 19th and early 20th centuries, populations were hit hard by plume hunters as their feathers were collected to make ladies hats. The millinery trade was the cause of decline of many species including snowy egrets, flamingos, roseate spoonbills, birds of paradise, terns, gulls and more.
This was the first of a one-two punch for the species. From the problem of hats, the birds soon faced the problem of a highway which brought development and more human pressures to its nesting habitat. According to Center for Biological Diversity:
The Migratory Bird Treaty Act of 1916 ended the [plume hunting] threat, but the least tern plummeted again some decades later due to growing development and recreational pressures which destroyed habitat, disturbed birds, and increased predation by introduced and native species. The construction of the Pacific Coast Highway brought all these threats to much of California's coast. By the 1940s, terns were gone from most beaches of Orange and Los Angeles counties and were considered sparse elsewhere. To avoid humans, some tern colonies nest at more inland mudflat and dredge fill sites, which appears to make them more susceptible to predation by foxes, raccoons, cats and dogs.
The California least tern was added to the Endangered Species List in 1970 and since then has been making a slow but steady recovery thanks to the protections offered by the listing.
The species still has a long way to go before it is stable enough to be considered for delisting, but those who love California least terns are encouraged each spring and summer when the birds return for the nesting season and raise the next generation of the species.