PREDATION BY KELP GULLS LARUS-DOMINICANUS AT A MIXED-SPECIES COLONY OF ROYAL TERNS STERNA-MAXIMA AND CAYENNE TERNS STERNA-EURYGNATHA IN PATAGONIA

We studied the predator-prey interaction between Kelp Gulls Larus domi nicanus and Royal Terns Sterna maxima and Gayenne Terns Sterna eurygna tha at Punta Leon, Argentina, from 1990 to 1992, Kelp Gulls were the m ain predator of tern eggs in an 3 years, accounting for 99% of observe d predations (n = 454). A total of 99% of predations occurred on perip heral tern nests, Total predation rate on both tern species was highes t in 1990 and varied significantly between years, with between 0.01 an d 0.03 cases of predation per hour, Predation rates in all years were significantly higher on Cayenne than on Royal Terns probably because t he latter were significantly more aggressive towards gulls than were C ayenne Terns. Royal and Cayenne Terns displayed aggressively towards g ulls at least once in 64% and 10% of the observation periods, respecti vely Defence behaviour, such as attacking and chasing the predator, wa s used more frequently by Royal than Cayenne Terns (13.2% v 3.6%). Pre dation on tern eggs was restricted to only a few Kelp Gulls which nest ed close to the tern colony periphery and 50% of the eggs that were st olen were taken by only two pairs during 1991 and 47% by three pairs i n 1992, This suggests that the current expansion of the Kelp Gull numb ers at Punta Leon might not necessarily result in a proportional incre ase in predation rate on both tern species, Stealing eggs from the gro und was more successful than from the air (77% v 17%), The nesting str ategy shown by Royal and Cayenne Terns at Punta Leon appears to be an important factor which reduced the impact on their breeding success of high potential predation pressure by gulls.