Abstract
Studying the movements of oceanic migrants has been elusive until the advent of several tracking devices, such as the light-level geolocators. Stable isotope analysis (SIA) offers a complementary approach to infer areas used year-round, but its suitability in oceanic environments remains almost unexplored. To evaluate SIA as a tool for inferring movements of oceanic migrants, we sampled an oceanic seabird, the Bulwer’s petrel, Bulweria bulwerii, in four breeding colonies spread along its Atlantic distribution. We first studied the species moulting pattern from 29 corpses collected in the colonies. Secondly, based on this moult knowledge, we selected three feathers from tracked birds to infer their breeding and non-breeding grounds using SIA: the 1st primary (P1), the 8th secondary (S8) and the 6th rectrix (R6) feathers. Birds migrated to two main non-breeding areas, the Central or the South Atlantic Ocean. P1 showed similar isotopic values among petrels from different breeding colonies, suggesting this feather is replaced early in the non-breeding period in a common area used by most birds, the Central Atlantic. S8 and R6 feathers correctly assigned 92% and 81%, respectively, of the birds to their non-breeding areas, suggesting they were replaced late in season, when birds were settled in their main non-breeding grounds. Our results showed that the isotopic baseline levels of the Central and South Atlantic are propagated through the food web until reaching top predators, suggesting these ratios can be used to infer the movement of long-distance migrants among oceanic water masses.
Highlights
Marine megafauna provide insights into the physical and biological processes occurring in the ocean and are generally considered good indicators of the health and structure of marine ecosystems as well as sensitive to human impacts [1,2,3]
Isotopic values of δ15N of feathers from 29 corpses revealed that variability in P1, P3 and P5 feathers was relatively low for the four colonies, when compared with that in P7, P10, S1, S8 or S12
The same low variability in P1 between colonies was detected in tracked birds regardless of the non-breeding areas, while the variability increased for the S8 and the R6 and showed differences between non-breeding areas (S1 Fig)
Summary
Marine megafauna provide insights into the physical and biological processes occurring in the ocean and are generally considered good indicators of the health and structure of marine ecosystems as well as sensitive to human impacts [1,2,3]. Megafauna, in particular seabirds, are often used to help identifying major hotspots relevant for marine biodiversity and for setting conservation priorities at sea, and there is an increasing need to know their year-round movements. This has boosted a plethora of tracking studies, most of them have been carried out on terrestrial species or on large pelagic species. Inferring movements of oceanic migrants was supported by a PhD grant from the University of Barcelona (D/134000500/P2017G/G00) and R. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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