Year-round movements of a small seabird and oceanic isotopic gradient in the tropical Atlantic

Despite the proliferation of seabird tracking studies, there is a relative paucity of studies on small tropical seabirds. We present for the first time the distribution and movements of the little-known Boyd's shearwater Puffinus boydi, a Procellariiform endemic to the Cape Verde Islands. We tracked 28 birds from 2 breeding sites (Ilheu Raso and Ilheu de Cima) with geolocator loggers from 2007 to 2012. We also analysed stable isotopes of carbon and nitrogen in the 1st primary (P1), the 6th rectrice (R6) and the 1st (S1) and 8th (S8) secondary feathers to reveal moulting pattern and oceanic isotopic gradients. Birds migrated on average 1450 km westward, to the central Atlantic Ocean (5 to 15 degrees N, 30 to 40 degrees W), where they stayed on average 114 d, from May to August. Boyd's shearwaters exploited oceanic waters year-round and showed delta C-13 values similar to other oceanic seabird species and delta N-15 values indicating the lowest known trophic level among all central Atlantic seabirds. Isotope values in flight feathers suggest most animals moult their P1 and R6 around the breeding ground, whereas all birds moult S1 and S8 at the non-breeding quarters. Correlations of delta C-13 and delta N-15 values from S8 with the longitude of the non-breeding area indicate the existence of large-scale isotopic gradients matching those known at baseline levels. Combining geolocator tracking and stable isotope analyses in feathers not only allowed us to describe in detail the annual life cycle and distribution of the species, but also the oceanic isotopic gradients in the tropical Atlantic.