| Summary: | Dissertação de mestrado em Ecologia, apresentada ao Departamento de Ciências da Vida da Faculdade de Ciências e Tecnologia da Universidade de Coimbra Changes in marine productivity have been correlated with fluctuations in large scale atmospheric conditions driven by global indices such as the North Atlantic Oscillation (NAO). During the winter of 2009/2010 the second lowest NAO index was recorded leading to detrimental conditions that affected productivity levels in the Northeast Atlantic. The Cory’s Shearwater Calonectris borealis is a plastic pelagic top predator which due to its status as sentinel of the marine environment can indicate qualitative changes at various levels of the food web. During the chick-rearing seasons of 2011, 2012 and 2014 we examined the at-sea foraging behavior of Cory’s Shearwaters breeding in two islands of the Northeast Atlantic with contrasting productivity patters: Berlengas, located on the rich upwelling are of the Portuguese shelf, and Cima Islet located in a poor oceanic environment in the Madeira Archipelago. During a year of low productivity Kernel estimations demonstrated that whereas Cima Islet’s birds expanded their home ranges and core foraging areas to further grounds, Berlengas’ birds maintained their distribution near the breeding colony. Under these conditions, birds from both islands increased significantly their foraging effort. MaxEnt modelling showed that once oceanographic conditions ameliorated from 2012-2014, all birds decreased significantly their foraging effort, and oceanic breeders concentrated their activity closer to their breeding grounds every year. Our study confirms that the Cory’s Shearwater possess very high flexibility in its foraging tactics, and that neither oceanic nor neritic populations disperse randomly from their breeding colonies to the open ocean even under conditions of environmental stochasticity. Instead, populations breeding in contrasting environments vary in their responses according to their strategies and to the changing levels ...
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