La Paz Bay, Baja California Sur, México, is located in the transition zone between two major biogeographic areas (based on fish and invertebrate distributions): the California region and the Cortez province (Briggs 1974 in Anderson 1983). These areas are characterized by patterns of climatic and latitudinal zonation that influence seabird distribution and breeding ranges (Anderson 1983), so data on species nesting in and near this transition zone may help clarify the factors governing these distributions. Within the bay, there are nine islands and extensive mangroves, constituting the main sites used by breeding waterbirds (Figure 1).