Reproductive ecology of Sterna hirundinacea from Cardos Island, Florianópolis, Santa Catarina, Brasil

Sterna hirundinacea is a migratory seabird which breeds on the Pacific Coast (from Peru to Chile) and on the Atlantic coast of South America from May to November in Espírito Santo (Brazil) to Tierra del Fuego (Argentina) during October-January. Despite its abundance and importance in South America, it remains understudied and several features of its ecology remain unknown. This work aims to characterize the reproductive ecology of South American terns on Cardos Island, Florianópolis, Brazil in the breeding seasons of 2003, 2005 and 2006. Daily checks of the colony were carried out and during these surveys, information on the total number of nests, egg measurements, diet composition and the success rates of incubation and breeding was obtained. The colony was formed in mid-May in 2003 and early April in other years, with the total number of nests ranging from 1,852 in 2006 to 2,486 in 2005. The average number of eggs per clutch ranged from 1.40 in 2006 to 1.19 in 2003, while the volume between 26.22 cm³ in 2005 to 26.98 cm³ in 2006. The hatching success in 2006 was estimated at 82.16%, 78.98% in 2003 and in 2005 the lowest value (75.44%) can be attributed to predation of hawks (Polyborus plancus) and black vulture (Coragyps atratus). The chicks emerged from July in 2003, June 2005 and 2006, where the reproductive success was 22.61%, 27.23% and 23.40% respectively. This study shows that Cardos Island has been constantly used as a breeding site by South American terns, therefore this island represents an important area for the conservation of this species. Such success can be attributed to the small size of the island, the low level of human disturbance, constant food supply and the absence of the Kelp gull (Larus dominicanus), the main predator of seabirds from the Brazilian coast.