Justification of Red List Category
Although this species may have a restricted range, it is not believed to 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.
Delany and Scott (2006) estimate the population at 25,000 individuals.
The population is suspected to be in decline owing to ongoing threats.
The Grey Gull breeds in the extremely arid Atacama Desert in northern Chile and is present as a non-breeding visitor throughout most of the Chilean coast, excluding the south, and the coast of Peru and Ecuador (del Hoyo et al. 1996).
This species feeds mainly and sometimes exclusively on mole crabs, but will also feed on fish, nereid worms and offal. It will also scavenge in harbours and follow fishing boats. Its breeding season ranges from November to January, breeding in the barren montane Atacama Desert between 35 and 100 km inland. It nests is a scrape in the sand, usually near rocks, but does not breed in years with a severe occurence of El Nino (del Hoyo et al. 1996).
Large El Niño events can cause complete breeding failure; in both 1982/3 and 1983/4 no chicks appeared to have fledged (Guerra et al. 1988). When food supplies are limiting, individuals move from nesting in the desert, where there is little predation but a journey of up to 100km to the coastal feeding grounds, to nesting in coastal locations despite enhanced predation risk (Aguilar et al. 2016). Dogs Canis familiaris are an additional, non-native source of predation capable of reducing reproductive success in a colony to zero, as noted for one of the novel coastal colonies where 40 nests were abandoned overnight with extensive signs of dog predation (Aguilar et al. 2016). However, the conditions in typical desert colonies are too extreme to support feral dogs, hence they are only likely to impact reproductive success in a significant way when breeding takes place close to the coast or urban locations. The species is also at risk from direct human interference. Large-scale mining operations have been proposed and in some cases undertaken close to active colonies; enhanced disturbance may contribute to colony abandonment and short-term reduced reproductive success. Large scale raids of colonies have been recorded and are likely to cause colony abandonment (Aguilar et al. 2012). Egg and chick collection appears to be a frequent occurrence when colony locations become known, and may contribute to colony switching demonstrated by the species (Aguilar et al. 2016).
Text account compilers
Calvert, R., Ekstrom, J., Bennett, S., Butchart, S., Martin, R., Stuart, A.
BirdLife International (2021) Species factsheet: Larus modestus. Downloaded from http://www.birdlife.org on 23/01/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 23/01/2021.