Seabirds have been proposed as suitable candidates for tracking and monitoring changes in marine systems (bioindicators). However, their suitability depends on our ability to link the large degree of environmental variability inherent to marine systems with a few, relevant, and accessible signals (biomarkers) informing on changes in their feeding behavior or reproductive performance.

We combined satellite remote-sensing records with stable isotope data (δ15N and δ13C) and breeding parameters (fledging success) spanning several years (2001−2014) to investigate the ecological responses to environmental variability by 2 sympatric seabirds inhabiting the western Mediterranean: Scopoli’s shearwater Calonectris diomedea and Cory’s shearwater C. borealis.

Both species showed similar annual variations in their stable isotopic composition, likely as a response to the trophic consequences of changes in the magnitude and timing of the annual peak in marine productivity (as proxied by satellite imagery of chlorophyll a concentrations). In contrast, no relevant responses were observed in their breeding performance, suggesting that their life-history strategy has evolved to constancy in breeding success, which diminishes its value as a biomarker of changes in marine productivity patterns.

Despite this limitation, combining remote sensing and stable isotopes in seabirds is a reliable and powerful tool for the early de – tection of fine-scale, climate-driven changes in marine productivity patterns and its cascading effects across communities and trophic levels, especially under the current scenario of ocean warming.