buenos aires province is one of the most disturbed regions of argentina due to human activities, yet has continental lagoons representing complex habitats which host abundant and diverse waterbirds, especially during the breeding season. our primary objective was to identify and survey breeding colonies of waterbirds in three continental lagoons of southeastearn buenos aires province, determining abundance, breeding phenology, and habitat selection of waterbirds. we identified four breeding colonies: one monospecific colony of cattle egret (bubulcus ibis), and three mixed-species colonies of brown-hooded gull (larus maculipennis) and grey-headed gull (larus cirrocephalus), cattle egret and white-faced ibis (plegadis chihi), and cocoi heron (ardea cocoi) and great white egret (egretta alba). los padres lagoon had the greatest species richness and most of the breeding colonies. colonies in different lagoons differed in their phenology of breeding activities, such as nest building, egg laying and chick rearing.