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 increasing, 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 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.
The global population has been estimated at between 115,500 and 118,500 individuals (Delany and Scott 2009).
The overall population trend is increasing, although some populations have unknown trends (Delany and Scott 2009). This species has undergone a small or statistically insignificant increase over the last 40 years in North America (data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007).
The Western Gull is found on the Pacific coast of North America, ranging from Vancouver Island (Canada) to the southern tip of Baja California (Mexico), and breeding from north-west Washington (USA) to central Baja California (Burger et al. 2018).
This species is essentially confined to the coast, staying on a few kilometres inland. It has a very varied diet, including marine fish and invertebrates, eggs, chicks and adults of seabirds, carrion, spawning salmon, and it may attack and kill newborn seal pups. Some males may establish feeding territories in alcid or cormorant colonies feeding mainly by predation, piracy and scavenging, occupying the same territory year after year. It also drops shellfish on to rocks to break them. It lays from late April or early May, and later in the north, nesting on barren substrates in colonies on rocky islets with some herbaceous cover and gravelly beaches. Some populations are migratory whilst others are sedentary, and individuals tend to disperse depending on food availability (del Hoyo et al. 1996).
At present no threats appear to be impacting populations of this species sufficiently to be driving any level of decline.
Text account compilers
Calvert, R., Fjagesund, T., Ekstrom, J., Butchart, S.
BirdLife International (2022) Species factsheet: Larus occidentalis. Downloaded from http://www.birdlife.org on 27/06/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 27/06/2022.