Predation by Kelp Gulls Larus-Dominicanus at a Mixed-Species Colony of Royal Terns Sterna-Maxima and Cayenne Terns Sterna- Eurygnatha in Patagonia

YORIO, P.; QUINTANA, F.

IBIS

Año: 1997 vol. 139 p. 536 - 541

We studied the predator-prey interaction between Kelp Gulls Larus dominicanus and Royal and Cayenne terns Sterna maxima and S. eurygnatha at Punta León, Argentina, during 1990 to 1992. Kelp Gull were the main predator of tern eggs in all years, accounting for 99% of observed predations (n = 454). A total of 99% of predations occurred on peripheral tern nests. Total predation rate on both tern species was highest in 1990 and varied significantly between years, with values that varied between 0.01 and 0.03 cases of predation per hour. Predation rates in all years were significantly higher on Cayenne than Royal terns probably because Royal Terns were significantly more aggressive toward gulls than Cayenne Terns. Royal and Cayenne Terns displayed aggressively towards gulls at least once in 64% and 10% of the observation periods, respectively. Defence behaviour such as attacking and chasing the predator were more fre­quently used by Royal than Cayenne Terns (13.2% v 3.6%). Predation on tern eggs were made by only a few Kelp Gulls nesting close to the tern colony periphery, with 50% of eggs stolen by only two pairs during 1991 and 47% by three pairs in 1992. This sug­gests that the current expansion of the Kelp Gull population at Punta León might not necessarily result in a proportional increase in predation rate on both tern species. Stealing eggs from the ground was more successful than from the air (77% v 17%). The nesting strategy shown by Royal and Cayenne Terns at Punta León appears to be an important factor which reduced the impact on the breeding success due to high predation pressure by gulls.