South American Tern Sterna hirundinacea


Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size may be moderately small to large, but it is not believed to approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Population justification
The population is estimated at 250,000-1,000,000 individuals.

Trend justification
The population is suspected to be in decline owing to disturbance of its nesting sites (del Hoyo et al. 1996).

Distribution and population

This species breeds on the coasts of southern Peru and Espírito Santo, east-central Brazil south to Tierra del Fuego, Argentina and Chile, as well as on the Falkland Islands (Malvinas) (Duffy et al. 1984, Schlatter 1984, Woods 1988, Antas 1991). In the austral winter, most birds breeding in the extreme south move north to Uruguay (where breeding remains unconfirmed). The species also winters north to Ecuador and Bahia, Brazil (Antas 1991). There are also large colonies on inshore islands in Guanabara Bay and off the Espírito Santo coast, Brazil (Antas 1991). The population on the Falkland Islands (Malvinas) was recently estimated at 6,000-12,000 pairs (Woods and Woods 1997). A colony exceeding 1,000 pairs was discovered in southern Peru in 1995 (del Hoyo et al. 1996), where it is fairly common (Clements and Shany 2001). However, although formerly locally common in Chile, only two breeding sites are remaining there (Damas Island and Inutil Bay near Porvenir), and extensive searches to locate former colonies between 1994 and 1997 were unsuccessful (S. Mickstein in litt. 1998).


The species is almost exclusively coastal, breeding on rocky or sandy beaches, cliff tops and small islands. Non-breeders frequent coastal waters, beaches, estuaries and harbours.The species feeds mainly on small fish and crustaceans and probably also takes insects. Laying occurs in April-June in Brazil, early November in northern Argentina and early December in southern Argentina. Two to three eggs are laid (del Hoyo et al. 1996).


The primary threat faced by this species is thought to be disturbance due to tourism, with a breeding colony at Lake Calafquen apparently deserted due to disturbance from bathing tourists and water bikes (S. Mickstein in litt. 1998). Tourism has also increased markedly on the Argentinean coast, and this has presumably had a detrimental effect on at least some breeding colonies (del Hoyo et al. 1996). Visitors to South American Tern colonies in Argentina have been recorded trampling nests and causing colony abandonment, and even driving 4x4 vehicles through nesting sites (Yorio et al. 2001). Egg collecting has previously been a problem at colonies in Espírito Santo, Brazil (Antas 1991), and may have contributed to the decline of the Chilean population (S. Mickstein in litt. 1998).


Text account compilers
Calvert, R., Malpas, L., Martin, R., Stuart, A., Butchart, S., Ekstrom, J.

Mickstein, S.

Recommended citation
BirdLife International (2021) Species factsheet: Sterna hirundinacea. Downloaded from on 18/10/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from on 18/10/2021.