NT
Elegant Tern Thalasseus elegans



Justification

Justification of Red List Category
This species is considered Near Threatened as it has a restricted breeding range, with more than 90% of the breeding population being restricted to a single island. It is also subject to large population fluctuations in response to climatic effects, and could be negatively affected by climate change, human intrusions and overfishing.

Population justification
The estimated population is 51,000-90,000 individuals (Kushlan et al. 2002) with up to 95% breeding on Isla Rasa in the Gulf of California (Velarde and Anderson 1994). At least three other Mexican islands are used at least occasionally (Velarde and Anderson 1994). In addition, small populations breed on Bolsa Chica (50-4,000 pairs, first recorded in 1987) and in San Diego bay (500-800 pairs) (Velarde and Anderson 1994, Gochfeld and Burger 1996, E. Verlarde in litt. 1998, B. Tershy and B. Keitt in litt. 1999).

Trend justification
The population is suspected to undergo dramatic fluctuations in response to El Niño Southern Oscillation events and subsequent fluctuations in fish populations (Velarde and Anderson 1994, B. Tershy and B. Keitt in litt. 1999).

Distribution and population

Thalasseus elegans breeds along the Pacific coast from south California, USA, to Baja California and the Gulf of California, Mexico (Howell and Webb 1995a, AOU 1998). Up to 95% of the population is thought to breed on Isla Rasa in the Gulf of California (Velarde and Anderson 1994). At least three other Mexican islands are used at least occasionally (Velarde and Anderson 1994). In addition, small populations breed on Bolsa Chica and in San Diego bay, California (Velarde and Anderson 1994, Gochfeld and Burger 1996, E. Verlarde in litt. 1998, B. Tershy and B. Keitt in litt. 1999). Non-breeding birds summer from California to Costa Rica (AOU 1998). Birds winter from Guatemala to El Salvador, Nicaragua, Costa Rica, Panama, Colombia, Ecuador, Peru and Chile (AOU 1998). There are significant population fluctuations, probably caused by the effects of El Niño Southern Oscillation (compounded by over-fishing) on prey abundance and consequently breeding success (E. Verlarde in litt. 1998, B. Tershy and B. Keitt in litt. 1999). Only the Isla Rasa colony breeds every year (B. Tershy and B. Keitt in litt. 1999), but fluctuations are considerably less than one order of magnitude.

Ecology

This species breeds on flat rocky areas and is strongly tied to the coast. It forages in inshore waters, estuarine habitats, salt ponds and lagoons, with some individuals venturing further offshore in the non-breeding season.

Threats

Declines in the fish stocks (e.g. sardines) due to overfishing, as well as fluctuations in stock due to El Niño events and sea surface temperature anomalies, cause dramatic fluctuations in the numbers of Elegant Terns, as well as large-scale migrations to new nesting sites. Previously, Elegant terns have shown high fidelity to breeding on Isla Rasa, with 90% of the population breeding on this island prior to 2000, even in adverse years. More recently there has been no successful nesting on Isla Rasa in 2003, 2009, 2010 or 2014 with each of these years being preceded by seasons of particularly high fishing effort. Warm sea surface temperature anomalies are thought to be the main factor determining the proportion of the tern population nesting away from Isla Rasa and the surrounding region, but fishing effort and total landings during the previous year also have a significant impact (Velarde et al. 2015). Currently, when the sea surface temperature anomaly around Isla Rasa is over 1°C, more than 70% of total Elegant tern nesting population is observed in three Californian colonies, especially in San Diego, compared to less than 20% in years with a sea surface temperature anomaly of less than 1°C (Velarde et al. 2015). Although human disturbance is not currently a significant threat to this species, as Elegant Terns move to nesting in more populated areas, such as San Diego, the impact of disturbance on nesting birds may increase.

Rats Rattus spp. were a problem on Isla Rasa until their successful eradication in 2009, upon which numbers of birds increased five-fold (Samaniego-Herrera et al. 2011). Rats are unlikely to return to Isla Rasa, but as Elegant Terns move to new, less protected, breeding locations on mainland they may be vulnerable to future impacts of invasive species such as rats. Historically, the Isla Rasa colony was nearly eliminated by eggers. This became illegal in 1960s and ended early 1980s (Velarde et al. 2015), as such, it is unlikely to return to Isla Rasa, but may pose a threat to new breeding colonies nesting in less well protected areas.

Conservation actions

Conservation Actions Proposed
Monitor population trends throughout the breeding range. Research links between climate, fisheries, prey availability and breeding success. Ensure continued effective protection of all breeding colonies.

Acknowledgements

Text account compilers
Gilroy, J., Butchart, S., Benstead, P., Martin, R., Moreno, R., Sharpe, C.J., Stuart, A., van der Merwe, N.

Contributors
Verlarde, E., Tershy, B., Keitt, B.


Recommended citation
BirdLife International (2019) Species factsheet: Thalasseus elegans. Downloaded from http://www.birdlife.org on 18/11/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 18/11/2019.