Thalasseus elegans

Formerly (AOU 1983, 1998) included in the genus Sterna but separated on the basis of genetic data that correspond to plumage patterns (Bridge et al. 2005).

The elegant tern is known to breed at only five sites in southern California and northwestern Mexico and is therefore vulnerable to stochastic events that could extirpate populations. Historically, populations have been negatively affected by introduced predators and human disturbance although predation by non-native invasive species has been largely eliminated. Overall protection has improved with new colonies having become established in recent decades. There is evidence this species' population has increased markedly in past decades.

The elegant tern is currently known from five extant breeding colonies including (in the U.S.), the Bolsa Chica Ecological Reserve, Orange County, the Los Angeles Harbor, Los Angeles County, breeding occurrences in both north and south San Diego Bay, and (in Mexico) Isla Montague, in the Colorado River delta, Gulf of California, and Isla Rasa, Gulf of California (Burness et al 2020). Detections of this species as a non-breeding vagrant in the U.S. Pacific Northwest, U.S. Eastern Seaboard, and in Europe have increased dramatically in recent decades (GBIF 2022). Veit et al (2021) hypothesize this apparent increase in the total population size is a response to removal of nest and egg predators from Isla Rasa in 1995 which supports over 90% of the known breeding population. During the nonbreeding season, most birds are found in South America, along the Pacific coast mainly from Ecuador to central Chile, with small numbers detected northward to central Mexico (Howell and Webb 1995, Burness et al 2020). After breeding, individuals disperse northward along the Pacific coast as far as northern California and occasionally to northern British Columbia (Burness et al. 2020).

Burness et al (2020) identify five current nesting colonies which for purposes of this assessment are considered the extant elegant tern occurrences.

Past threats included non-native nest and egg predators on breeding islands (Burness et al 2021) although this threat has been largely eliminated. Human disturbance at nest sites, especially in California, has been identified as a threat factor. The effects of climate change on prey species and possibly sea level rise associated with warmer global conditions may depress populations in some years. This species appears to be highly adaptable to changing conditions as evidenced by population responses following El Nino events and fishing pressure changes (Velarde et al 2015).