Heermann's Gull Larus heermanni Scientific name definitions

Named after Adolphus Lewis Heermann, the 19^th century explorer and naturalist, this medium-sized gull is unlike any other North American gull in both appearance and migration pattern. The white head of breeding adults transitions to slate gray at the neck and down to the belly, and the red bill is tipped with black. Genetic evidence (1 Pons, J. M., A. Hassanin, and P. A. Crochet (2005). Phylogenetic relationships within the Laridae (Charadriiformes: Aves) inferred from mitochondrial markers. Molecular Phylogenetics and Evolution 37:686–699. Close ) shows that Heermann's Gull belongs to the 'White-headed species' group of the Holarctic region. However, its exact position within this group is not well-resolved (2 Sonsthagen, S. A., R. E. Wilson, R. T. Chesser, J.-M. Pons, P.-A. Crochet, A. Driskell, and C. Dove (2016). Recurrent hybridization and recent origin obscure phylogenetic relationships within the ‘white-headed’ gull (Larus sp.) complex. Molecular Phylogenetics and Evolution 103:41–54. Close ).

Gregarious by nature, Heermann's Gull forms breeding colonies on arid islands, mainly in the Gulf of California, Mexico, and a few have been recorded nesting on islands along the Pacific coasts of Mexico and California (United States). The nesting season extends from March through July, depending on the location of the colony. The largest colony is on Isla Rasa, where an estimated 90–95% of the world population breeds. After breeding, most adults and young migrate from the Gulf of California to the Pacific coast and spread south to southern Mexico and north to southern British Columbia, with peak numbers arriving in Canada during July and August. These northern individuals then retreat south to winter along the coast only from California south, with some nonbreeding immatures remaining along the coast year-round.

During the nonbreeding season, some individuals feed along beaches, sheltered bays and harbors, rocky promontories, and kelp beds, but the majority are maritime and pursue schools of herring many miles offshore. During the nesting season, in the Gulf of California, they feed mainly on small pelagic fish, most importantly Pacific sardine (Sardinops sagax), northern anchovy (Engraulis mordax), and Pacific mackerel (Scomber japonicus) (3 Velarde, E., E. Ezcurra, M. A. Cisneros-Mata, and M. F. Lavín (2004). Seabird ecology, El Niño anomalies, and prediction of sardine fisheries in the Gulf of California. Ecological Applications 14:607–615. Close , 4 Velarde, E., E. Ezcurra, and D. W. Anderson (2013). Seabird diets provide early warning of sardine fishery declines in the Gulf of California. Scientific Reports 3:1332. Close , 5 Velarde, E., E. Ezcurra, and D. W. Anderson (2015). Seabird diet predicts following-season commercial catch of Gulf of California Pacific Sardine and Northern Anchovy. Journal of Marine Systems 146:82–88. Close ). They often join mixed-species assemblages of feeding cormorants (Phalacrocorax spp.), boobies (Sula spp.), and Brown Pelican (Pelecanus occidentalis), and kleptoparasitize food, particularly from Brown Pelicans, by snatching fish directly from the pelican’s pouch. Nests consist of simple scrapes on the ground sometimes lined with debris. Clutch size ranges from 1–3 eggs, which varies according to food abundance and female age (6 Vieyra, L., E. Velarde, and E. Ezcurra (2009). Effects of parental age and food availability on the reproductive success of Heermann’s Gulls in the Gulf of California. Ecology 90:1084–1094. Close ). After 28 days of incubation, semiprecocial chicks hatch and fledge at approximately 45 days of age.

Historically, Heermann's Gull was harvested by Mexican fisherman and Native American egg collectors; an estimated 50,000 eggs were removed during one breeding season at Isla Rasa (7 Danemann, G., E. Ezcurra, and E. Velarde (2008). Conservación ecológica. In Bahía de Los Angeles: Recursos Naturales y Comunidad, Linea Base 2007 (G. Danemann and E. Ezcurra, Editors). SEMARNAT/INE/Pronatura NW/San Diego Natural History Museum, Mexico City, Mexico. pp. 695–729. Close ). Other threats have included high levels of pesticide contaminants in body tissues accumulated through diet during the 1980s (8 Porter, R. D., and M. A. Jenkins (1988). Pollutants and eggshell thinning in peregrines and their prey in the Baja California region. In Peregrine Falcon Populations, Their Management and Recovery (T. J. Cade, J. H. Enderson, C. G. Thelander, and C. M. White, Editors). The Peregrine Fund, Inc., Boise, ID, USA. pp. 413–421. Close ), direct competition with humans for Pacific sardine (3 Velarde, E., E. Ezcurra, M. A. Cisneros-Mata, and M. F. Lavín (2004). Seabird ecology, El Niño anomalies, and prediction of sardine fisheries in the Gulf of California. Ecological Applications 14:607–615. Close ), human disturbance at breeding colonies (9 Anderson, D. W., J. E. Mendoza, and J. O. Keith (1976). Seabirds in the Gulf of California: A vulnerable international resource. Natural Resources Journal 16:483–505. Close ), and introduction of nonnative vertebrates such as black rat (Rattus rattus) and house mouse (Mus musculus). However, more recent studies have detected lower levels of pesticides in body tissues (Anderson et al., unpublished data), and nonnative rodents were eradicated from Isla Rasa in 1995 (7 Danemann, G., E. Ezcurra, and E. Velarde (2008). Conservación ecológica. In Bahía de Los Angeles: Recursos Naturales y Comunidad, Linea Base 2007 (G. Danemann and E. Ezcurra, Editors). SEMARNAT/INE/Pronatura NW/San Diego Natural History Museum, Mexico City, Mexico. pp. 695–729. Close ). With establishment of Isla Rasa as a seabird sanctuary by the Mexican Government in 1964, protection has been afforded to the most important breeding colony. From an estimated low of 55,000 pairs in 1975, the global population is currently estimated at 350,000 individuals. This increase may account for recent nesting along the California coastline and an increase in extralimital records in North America in recent decades.

Long-term research at Isla Rasa, Gulf of California, Mexico, has provided the only detailed information on diet (10 Velarde, E., M. S. Tordesillas, L. Vieyra, and R. Esquivel (1994). Seabirds as indicators of important fish populations in the Gulf of California. California Cooperative Oceanic Fisheries Investigations Report 35:137–143. Close , 3 Velarde, E., E. Ezcurra, M. A. Cisneros-Mata, and M. F. Lavín (2004). Seabird ecology, El Niño anomalies, and prediction of sardine fisheries in the Gulf of California. Ecological Applications 14:607–615. Close , 4 Velarde, E., E. Ezcurra, and D. W. Anderson (2013). Seabird diets provide early warning of sardine fishery declines in the Gulf of California. Scientific Reports 3:1332. Close , 11 Velarde, E., E. Ezcurra, and D. W. Anderson (2014). Seabirds and pelagic fish abundance in the Midriff Island Region. In Conservation Science in Mexico´s Northwest: Ecosystem Status and Trends in the Gulf of California (E. Wehncke, J. R. Lara-Lara, S. Alvarez-Borrego, and E. Ezcurra, Editors), University of California Institute for Mexico and the United States, México City, Mexico. pp. 237–248. Close , 5 Velarde, E., E. Ezcurra, and D. W. Anderson (2015). Seabird diet predicts following-season commercial catch of Gulf of California Pacific Sardine and Northern Anchovy. Journal of Marine Systems 146:82–88. Close ). Aspects of egg water loss, temperature regulation, and oxygen consumption in both adults and embryos on the arid islands have been documented (12 Bennett, A. F., and W. R. Dawson (1979). Physiological responses of embryonic Heermann's Gulls to temperature. Physiological Zoology 52:413–421. Close , 13 Rahn, H., and W. R. Dawson (1979). Incubation water loss in eggs of Heermann's and Western gulls. Physiological Zoology 52:451–460. Close , 14 Ellis, H. I., and J. Frey (1984). Energetics of thermoregulation in Heermann's Gull and a test of a latitudinal gradient of metabolism. American Zoology 24:138. Close ). Other detailed studies on Isla Rasa include density estimates of breeding colonies (15 Velarde, E. (1999). Breeding biology of Heermann's Gulls on Isla Rasa, Gulf of California, Mexico. Auk 116:513–519. Close ), breeding biology (16 Boswall, J., and M. Barrett (1978). Notes on the breeding birds of Isla Rasa, Baja California. Western Birds 9:93–108. Close , 17 Velarde, E. (1983). Egg and chick predation and breeding success of the Heermann's Gull at Isla Rasa, Baja California, Mexico: Perspectives for seabird rational exploitation. In Proceedings of Jean Delacour/International Foundation for the Conservation of Birds Symposium on Breeding Birds in Captivity 1983. Los Angeles, CA, USA. pp. 155–169. Close , 15 Velarde, E. (1999). Breeding biology of Heermann's Gulls on Isla Rasa, Gulf of California, Mexico. Auk 116:513–519. Close , 18 Urrutia, L. P., and H. Drummond (1990). Brood reduction and parental infanticide in Heermann's Gull. Auk 107:772–774. Close ), behavioral postures associated with incubation and thermoregulation (19 Bartholomew, G. A., and W. R. Dawson (1979). Thermoregulatory behavior during incubation in Heermann's Gulls. Physiological Zoology 52:422–437. Close ), interactions with other species (20 Velarde, E. (1992). Predation of Heermann's Gull (Larus heermanni) chicks by Yellow-footed Gulls (Larus livens) in dense and scattered nesting sites. Colonial Waterbirds 15:8–13. Close , 21 Velarde, E. (1993). Predation of nesting larids by Peregrine Falcons at Rasa Island, Gulf of California, Mexico. Condor 95:706–708. Close ), population estimates at major breeding sites (22 Velarde, E., and D. W. Anderson (1994). Conservation and management of seabird islands in the Gulf of California: Setbacks and successes. In Seabirds on Islands: Threats, Case Studies and Action Plans (D. N. Nettleship, J. Burger, and M. Gochfeld, Editors). Birdlife Conservation Series 1, Birdlife International, Cambridge, United Kingdom. pp. 229–243. Close , 5 Velarde, E., E. Ezcurra, and D. W. Anderson (2015). Seabird diet predicts following-season commercial catch of Gulf of California Pacific Sardine and Northern Anchovy. Journal of Marine Systems 146:82–88. Close ), demographic history (23 Ruiz, E. A., E. Velarde, and A. Aguilar (2017). Demographic history of the Heermann´s Gull (Larus heermanni) from late Quaternary to present: Effects of past climate change in the Gulf of California. Auk: Ornithological Advances 134: 308–316. Close ), life history strategies (24 Velarde, E., and E. Ezcurra (2018). Are seabirds´ life history traits maladaptive under present oceanographic variability? The case of Heermann´s Gull Larus heermanni (Charadriiformes: Laridae). Condor: Ornithological Applications 120: 388–401. Close ), and human impact at breeding colonies (19 Bartholomew, G. A., and W. R. Dawson (1979). Thermoregulatory behavior during incubation in Heermann's Gulls. Physiological Zoology 52:422–437. Close , 25 Anderson, D. W., and J. O. Keith (1980). The human influence on seabird nesting success: Conservation implications. Biological Conservation 18:65–80. Close , 18 Urrutia, L. P., and H. Drummond (1990). Brood reduction and parental infanticide in Heermann's Gull. Auk 107:772–774. Close , 20 Velarde, E. (1992). Predation of Heermann's Gull (Larus heermanni) chicks by Yellow-footed Gulls (Larus livens) in dense and scattered nesting sites. Colonial Waterbirds 15:8–13. Close , 15 Velarde, E. (1999). Breeding biology of Heermann's Gulls on Isla Rasa, Gulf of California, Mexico. Auk 116:513–519. Close ).