Justification of Red List Category
This species has a very 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). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size may be small, 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.
The population is estimated to number 1,000-10,000 individuals, roughly equating to 670-6,700 mature individuals.
The overall population trend is stable, although some populations have unknown trends (Delany and Scott 2006).
This species breeds in south-east Brazil and Uruguay south to Patagonia, though rarely in Santa Cruz, Argentina, and in Chile (from Aconcaqua to Llanquihue). In winter, the species moves north as far as southern Peru on the west coast, and Rio de Janiero, Brazil, on the east coast (del Hoyo et al. 1996).
This species frequents fresh and saline wetlands, both coastal and inland. Its diet includes small fish and insects In Chile, it mainly feeds on pejerrey. It forages over shallow clear waters on the edge of lagoons, rivers and estuaries, and plunge-dives for fish. It will also feed on plouged fields. Breeding appears to occur from October to December on vegetated lagoons, mainly in marhses, but also on dykes and islands in saline lagoons. Adults will vigorously attack and even strike intruders (del Hoyo et al. 1996).
The species only nests in large wetlands, suggesting habitat loss, especially through abstraction of surface water for agricultural use, may be a potentially serious problem (del Hoyo et al. 1996).
Text account compilers
Calvert, R., Ekstrom, J., Harding, M., Butchart, S., Martin, R.
BirdLife International (2020) Species factsheet: Sterna trudeaui. Downloaded from http://www.birdlife.org on 13/07/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 13/07/2020.