We analyzed Royal Tern abundance and its seasonal variation along 2005 at Cananéia-Iguape-Ilha Comprida estuary, southern São Paulo, Brazil. Royal Terns were found throughout the year at the study area. Censuses were made from January to December 2005, on a weekly basis. A total of 48 census were realized in each five areas of the estuary. The results show that Royal Tern is a common species at the estuary and the number of individuals varied among the sites. The largest numbers were found in winter, agreeing with the nesting season of the local population and, possibly, the influx of birds from southern Brazil and Argentina. During late spring and early summer, the numbers decreased

Distribution of the Royal Tern (Thalasseus maximus) during 2005 in the Cananéia-Iguape-Ilha Comprida estuary