BREEDING BIOLOGY OF ROYAL AND CAYENNE TERNS AT A MIXED-SPECIES COLONYIN PATAGONIA

We studied the breeding biology of Royal (Sterna maxima) and Cayenne t erns (S. eurygnatha) at Punta Leon, Patagonia, during 1989-93. In all year-a, both species bred in a single colony with their nests intermin gled. Colony size varied between 1,150 and 1,830 pairs. Colony formati on occurred during the first or second week of October. Nesting densit ies were relatively high (9.8 to 11.1 nests/m(2)). Both species starte d to lay eggs during the second and third week of October in all years . During 1993, clutch size for Royal and Cayenne terns was 1.3 +/- 0.5 and 1.1 +/- 0.3, respectively. The length of the incubation period wa s 25.7 +/- 0.9 and 29.1 +/- 3 days for Royal and Cayenne terns, respec tively. During 1993, hatching success was 0.72 +/- 0.34 and 0.68 +/- 0 .42 chicks per nest for Royal and Cayenne terns, respectively. Egg los ses (45.3%) were mainly from predation. First-hatched chicks were obse rved during mid November. Chick survival to 10 days of age was 90% and 66.7% for Royal and Cayenne terns, respectively. At approximately 20 days of age, some tern chicks started to abandon the colony to form cr eches at the beach. Royal and Cayenne terns generally fed out of sight from the colony and preyed upon the same species, including Odonteste s spp., Stromateus brasiliensis. Engraulis anchoita, and Sprattus fueg uensis.