Does El Niño Affect Survival Rate of Mediterranean Populations of Cory's Shearwater?

Cory's Shearwater (Calonectris diomedea diomedea) survival rates were analyzed by means of capture histories of a small population of breeding birds banded from 1988 to 1998 in two caves on the Tremiti Islands, South Italy, to test the hypothesis that El Niño has a negative effect on Cory's survival rates. We tested the hypothesis of a relationship between Cory's survival rates and the ENSO cycle, modeling the survival and recapture probabilities by means of the program SURGE. After allowing for temporal variation in capture probabilities, we found that the variation of survival rates during our study period was in accordance with the ENSO cycle. We found no evidence of a negative effect of the warm episodes (El Niño) on the survival rates of Cory's Shearwater; instead, a lowering of survival rates were detected in connection with the cold episodes (La Niña). The main effect of La Niña over the range of GCory's Shearwater is cooler and wetter conditions along the Atlantic coasts and in South Africa, where the birds winter, but La Niña conditions may also strengthen the Atlantic hurricane season. We then constrained the survival rate by the number of intense hurricane days, producing a second, better-fitted model. The effects of the greater numbers of Atlantic hurricanes and storms may be a possible explanation of the lowering of Cory's survival rate observed during La Niña years.