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 is very large, and hence does not 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.
Wetlands International (2018) estimate the population to be 621,000 individuals.
This species has undergone a small or statistically insignificant decrease over the last 40 years in North America (data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007).
The breeding range of this species in in western North America from Northwest Territories (Canada) south to eastern California and Colorado (USA). It winters in coastal regions from south-west Canada to south-west Mexico (del Hoyo et al. 1996).
This species can be found on a variety of habitats, including coasts, estuaries, bays, mudflats and fields, breeding in open habitats usually on low rocky islands in freshwater and hypersaline lakes. Its breeding season begins in early May, laying in April and May in colonies. It feeds on insects, grubs, the eggs and young of birds, rodents, rubbish, grain and berries (del Hoyo et al. 1996).
Mono Lake, which holds one of the largest breeding populations, has undergone periods of reduced water level due to prolonged drought in recent years, resulting in breeding islands becoming accessible to coyotes Canis latrans and other mammalian predators that typically have little impact on the breeding population size (Nelson and Greiner 2014). This population appears to be undergoing a long-term decline, but populations in San Francisco bay have increased rapidly over the same period.
Text account compilers
Ekstrom, J., Butchart, S., Martin, R., Calvert, R., Stuart, A.
BirdLife International (2020) Species factsheet: Larus californicus. 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.