Facing extremes: Cory’s shearwaters adjust their foraging behaviour differently in response to contrasting phases of North Atlantic Oscillation

Abstract

Climate projections predict increases in the frequency and severity of extreme climate events over the next decades. Hence, phases of extreme climatic indices are emerging as one of the most dangerous effects of climate change, though their impacts on wildlife populations are still poorly understood. Here, we studied the foraging behaviour, body condition and breeding performance of a neritic (Berlenga Island) and oceanic (Corvo Island) population of Cory’s shearwaters (Calonectris borealis) in the mid-North Atlantic, during the two most positive and negative phases of North Atlantic Oscillation (NAO) reported in recent decades. We showed that during an extreme negative NAO phase, birds from Berlenga spent less time foraging and provided less food to their chicks, which subsequently grew more slowly and were in poorer body condition. In contrast, the opposite pattern was found during the strong positive NAO phase in this population. Interestingly, during the same extreme negative NAO phase, birds from Corvo were more successful in terms of their foraging and breeding performance, taking advantage of the enhanced productivity associated with the cyclonic eddies (negative sea surface height anomalies) which occurred close to the colony. However, when anticyclonic eddies (positive sea surface height anomalies) were prevalent near the colony during the strong positive NAO phase, birds travelled longer distances, which negatively impacted their own body condition and that of their chicks. Our study shows that populations breeding in neritic and oceanic areas of the eastern North Atlantic Ocean make contrasting foraging behavioural decisions in response to climate extremes and highlights the importance of mesoscale eddies for oceanic populations of pelagic seabirds.