DIFFERENTIAL RESPONSES OF TROPICAL ROSEATE TERNS TO AERIAL INTRUDERS THROUGHOUT THE NESTING CYCLE

We studied a small (63-pair) breeding colony of Roseate Terns (Sterna dougallii) at Culebra, Puerto Rico, in 1990 to identify disturbances caused by, and responses elicited by, aerial intruders and to determine the effect of predation on reproductive success of this threatened species. Laughing Gulls (Larus atricilla) were the most frequent cause of disturbance to nesting adults and captured at least two chicks. More intrusions occurred during the early stages of the breeding season and in the early morning than at other times. Defensive aggression in nesting Roseate Terns increased until the chick hatching stage, then decreased progressively through fledging. Neither the number of adults nor the number of chicks in the colony influenced aggressive responses against intruders as strongly as the nesting stage, suggesting that this population of Roseate Terns was most aggressive when the young were most vulnerable. Although our findings contrast with traditional parental investment theory, they are consistent with other studies of defensive aggression in seabirds. Most chick losses occurred during or shortly after hatching. Reproductive success of the colony was 0.25 +/- 0.50 chicks/pair and may have been low due to egg and chick mortality caused by predatory land crabs (Gecarcinus ruricola) that were abundant in the colony. Our findings suggest that although Roseate Terns responded appropriately to potential avian predators at this small Culebra colony, they were unable to successfully defend against certain ground predators (e.g., crabs).