Justification of Red List Category
This species is considered Near Threatened as it has a restricted breeding range, with 90% of the world population breeding on a single small island. Nesting populations fluctuate widely in response to climatic events, although these fluctuations are less than one order of magnitude. It is likely to be at high risk from catastrophic events or local anthropogenic changes, and should be carefully monitored.
Larus heermanni is estimated to have a population of 350,000 mature individuals (Partners in Flight 2019), ~92% of which nest on Isla Rasa, Mexico (Burger and Gochfeld 1996). Numbers on Isla Rasa are similar to estimates in the late 1960s, but there have been significant fluctuations with a low of 55,000 pairs in 1975 (Burger and Gochfeld 1996). There are further throughout to be 150 pairs on George Island, 1,500 on Cholluda (Burger and Gochfeld 1996), 4,000 on Cardonosa and 200 breeding individuals on San Il de Fonso (Velarde 1999) however, more recent surveys are required to confirm such figures.
The overall population trend is fluctuating (Delany and Scott 2006). This species has undergone a large and statistically significant increase over the last 40 years in North America (361% increase over 40 years, equating to a 46.6% increase per decade; data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007). These surveys however cover less than 50% of the species' range in North America. In the last eight years, almost total breeding failure has occurred in five nesting seasons. This is mainly due to environmental anomalies, that result in lack of food availability and, secondarily to overfishing of small pelagic fish (mainly Pacific sardine) their main food sources. An analysis of the trend of the population of this species is in process, to be able to predict the rate of population fluctuation over several years.
Larus heermanni has a population of approximately 283,000 to 300,000 breeding adults (Kushlan et al. 2002), 92% of which nest on Isla Rasa, Mexico (Burger and Gochfeld 1996). There are 150 pairs on George Island, 1,500 on Cholluda (Burger and Gochfeld 1996), 4,000 on Cardonosa and 200 breeding individuals on San Il de Fonso (Velarde 1999), with breeding south to Nayarit and islas San Benito and San Roque, and sporadically north to California, U.S.A. (Everett and Anderson 1991, Burger and Gochfeld 1996). Post-breeding dispersal occurs commonly to central California and in smaller numbers north to British Columbia, Canada, and south to Guatemala (Burger and Gochfeld 1996). Numbers on Isla Rasa are similar to estimates in the late 1960s, but there have been significant fluctuations with a low of 55,000 pairs in 1975 (Burger and Gochfeld 1996).
This coastal species breeds very synchronously and often at high densities (up to 110 nests/100m2) on remote rocky coasts and islets (Velarde 1992, 1999). It feeds largely within inshore waters and in the littoral zone, but also in oceanic waters surrounding the breeding islands.
Population fluctuations are probably caused by the effects of El Niño Southern Oscillation, compounded by over-fishing, on prey abundance and consequently breeding success, and even adult survival (Velarde et al. 1994, E. Velarde in litt. 1998, Velarde 1999). During strong El Niño years breeding success collapses to near zero, in comparison to around 0.5 in a typical year, and individual female mass is greatly reduced (Case et al. 2002). Ocean warming events and high sea surface temperature (SST) conditions greatly impact population growth rates in Larus heermanni. Under stable, normal SST conditions, population growth is estimated at ~4% however under high SST conditions, anomalously warm temperatures can result in a decline in population growth of up to 15%. Whilst the species can resist oceanographic anomalies of up to 1 event every 5 years without compromising population growth rates, the increasing frequency of anomalously warm SST conditions will likely facilitate increasingly rapid and severe population declines in the decades to come (Velarde and Ezcurra 2018).
About 90% of the global population exists on Isla Rasa, a basalt platform of 0.68 km2 and, as such, the occupied breeding range is very small. However, Isla Rasa was declared a sanctuary in 1964 (Anderson et al. 1976), and has been well-managed during the breeding season by resident biologists, who discourage egg-collecting and disturbance (E. Velarde in litt. 1998, B. Tershy and B. Keitt in litt. 1999). Commercial egg collection prior to the declaration of Isla Rasa as a sanctuary considerably reduced the population (Bowen et al. 2015). Any weakening of protection on the island may see an increase in egg collection once again, and there is little protection for breeding attempts on other islands, though these are only a small percentage of the population. Introduced black rats Rattus rattus and house mice Mus musculus were eradicated from Isla Rasa in 1995, and reproductive success in Heermann’s Gulls doubled in the areas where rodents were previously most prevalent (Bowen et al. 2015). It is likely that these invasive species impact breeding attempts on additional islands.
Heermann’s Gulls may be at risk from pollution, cadmium concentrations during two seasons were found to be above threshold levels for adverse impacts on reproduction and survival, and were the highest recorded in a study of 8 seabird species in coastal Sinaloa (Ceyca et al. 2016). The impact of this contamination is currently unknown.
Conservation and Research Actions Underway
No specific actions are known.
Conservation and Research Actions ProposedContinue to take active measures to protect breeding populations on islands, particularly Isla Rasa. Monitor population trends at breeding sites, particularly in response to climatic change or fluctuation.
Text account compilers
Martin, R., Everest, J., Stuart, A.
Benstead, P., Butchart, S., Gilroy, J., Keitt, B., Moreno, R., Sharpe, C.J., Tershy, B. & Velarde, E.
BirdLife International (2021) Species factsheet: Larus heermanni. Downloaded from http://www.birdlife.org on 26/11/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 26/11/2021.