We recorded eight species of birds in the Laridae family on the Ilha Comprida beach (southern São Paulo State, Brazil), during weekly censuses over 3 years (1999, 2000, and 2001). These species were grouped into three clusters by their abundance, permanence, and time of beach-flat use. The first cluster was formed by Larus dominicanus, Sterna eurygnatha, and Sterna maxima, which were in high abundance and present on the flat in all years. The second cluster included Sterna hirundo and Sterna hirundinacea. Sterna hirundo were seasonally abundant, and all individuals migrated to North America during the reproductive season. Sterna hirundinacea were always present for short periods. The third group included species of low abundance: Sterna superciliaris, Sterna trudeaui, and Larus maculipennis. The number of species and the number of individuals probably fluctuated because of migration or movement between neighboring beaches.

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