Foraging ecology of Cory’s shearwaters in different oceanic environments of the North Atlantic

The use of stable isotopes of animal tissues to infer diet and habitat selection has emerged as a powerful tool, particularly when combined with conventional dietary analysis to provide an integrated view of the assimilated diet. We used 3 techniques during 3 yr (2006 to 2008) on 6 different populations of Cory’s shearwatersCalonectris diomedea), ranging from neritic (Berlengas archipelago) to oceanic areas (Azores, Desertas and Selvagens archipelagos), to evaluate the trophic and foraging ecology of this species over the North Atlantic. We deployed data loggers to track foraging movements and feeding locations, and collected blood and diet samples from each individual after each foraging excursion. We also measured the isotopic signatures of the main prey species for each population. Analysis of stomach regurgitations showed thatTrachurus picturatuswas the main prey for populations exploiting oceanic environments (Azores and Desertas);Sardina pilchardusandBelone belonedominated the diet of birds feeding in neritic areas (Berlengas); andTrachurus trachurus,Scombersp. andExocoetus volitanswere important for birds exploiting both neritic and oceanic areas (Selvagens). Cephalopods (squid) were important for all populations. The birds’ blood δ13C signatures matched those of their main prey items. The blood δ13C signature was negatively correlated with the latitude of the main feeding locations of individuals and segregated populations exploiting neritic and shelf systems from those feeding in oceanic and seamount areas. Even the signatures of geographically different populations that fed in the same oceanic regions were similar. The δ15N signature was negatively correlated with the abundance of cephalopods in the diet. Results from a stable isotopic mixing model estimated higher proportions of cephalopods in the birds’ diet than proportions obtained by direct diet inferences, presumably due to the advanced digestion stage of cephalopods in the regurgitations. Our findings should have broader relevance to the study of foraging ecology of other top marine predators in the north Atlantic region.