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Range DescriptionThalasseus elegans breeds along the Pacific coast from south California, USA, to Baja California and the Gulf of California, Mexico (Howell and Webb 1995a, AOU 1998). The estimated population is 51,000-90,000 individuals (J. A. Kushlan et al. 2002) with up to 95% breeding on Isla Rasa in the Gulf of California (Velarde and Anderson 1994). At least three other Mexican islands are used at least occasionally (Velarde and Anderson 1994). In addition, small populations breed on Bolsa Chica (50-4,000 pairs, first recorded in 1987) and in San Diego bay (500-800 pairs), California (Velarde and Anderson 1994, Gochfeld and Burger 1996, E. Verlarde in litt. 1998, B. Tershy and B. Keitt in litt. 1999). Non-breeding birds summer from California to Costa Rica (AOU 1998). Birds winter from Guatemala to El Salvador, Nicaragua, Costa Rica, Panama, Colombia, Ecuador, Peru and Chile (AOU 1998). There are significant population fluctuations, probably caused by the effects of El Niño Southern Oscillation (compounded by over-fishing) on prey abundance and consequently breeding success (E. Verlarde in litt. 1998, B. Tershy and B. Keitt in litt. 1999). Only the Isla Rasa colony breeds every year (B. Tershy and B. Keitt in litt. 1999), but fluctuations are considerably less than one order of magnitude.