Justification of Red List Category
This species is considered Near Threatened as it has a highly 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.
Kushlan et al. (2002)
The overall population trend is fluctuating (Wetlands International 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). Note, however, that these surveys cover less than 50% of the species' range in North America. However, 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. One 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, USA (Everett and Anderson 1991, Burger and Gochfeld 1996). Post-breeding dispersal occurs commonly to central California, USA, 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). Until early this century fluctuations have been considerably less than one order of magnitude and therefore the species is not even close to qualifying as threatened for this reason. The occupied breeding range is very small, but 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). However, in the last decade increased frequency of warm oceanographic anomalies plus overfishing of the main forage fish that the species feeds on has dramatically reduced the breeding success and it is not yet clear how this will affect the population survival. Analysis are underway to produce predictive models that will help determine future status for the species.
Conservation and Research Actions UnderwayNo specific actions are known.
Conservation and Research Actions Proposed
Continue 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
Sharpe, C.J., Moreno, R., Gilroy, J., Butchart, S., Benstead, P.
Tershy, B., Velarde, E., Keitt, B.
BirdLife International (2018) Species factsheet: Larus heermanni. Downloaded from http://www.birdlife.org on 23/02/2018. Recommended citation for factsheets for more than one species: BirdLife International (2018) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 23/02/2018.