Foraging strategies of a generalist seabird species, the yellow-legged gull, from GPS tracking and stable isotope analyses
Generalist and opportunistic species, such as the yellow-legged gull Larus michahellis, can feed on a wide variety of food from both marine and terrestrial origins. This work evaluates the potential foraging strategies (terrestrial, mix and marine) of the yellow-legged gull during the breeding season, in Berlenga (39°24′55″N, 9°30′28″W) and Deserta (36°57′45″N, 7°53′29″W) Islands, Portugal, across 2011–2016. Stable isotope analyses (δ13C and δ15N) of plasma and blood cells were performed to estimate the proportion of individuals pursuing each strategy based on discrimination analyses. For that, GPS loggers were used to assess individual foraging destinations. Overall, this study discriminated well the foraging strategies adopted by gulls through stable isotope analyses (estimated error of 16.7%). Results indicated a variation in foraging strategies across years and between colonies. As expected, this variation was influenced by oceanographic conditions and availability of marine food sources nearby the colonies. The isotopic niche of yellow-legged gulls pursuing a marine strategy was much smaller than the niche of gulls with a terrestrial strategy, but surprisingly only slight smaller than the niche of gulls pursuing a mixed strategy. Gulls adopting a terrestrial strategy fed on a wide variety of foods, which greatly influenced the amplitude of the isotopic values, and respective isotopic niche width. On the other hand, gulls adopting a mix strategy might be very selective in the consumption of foods, taking great advantage of their potential plasticity on both marine and terrestrial environments. This study highlights an overall preference for the marine and mix strategies in yellow-legged gulls.
We would like to thank the Instituto da Conservação da Natureza e Florestas (ICNF) for permits and logistical support (lodging) to conduct this work. Special thanks to the wardens, Paulo Crisóstomo and Eduardo Mourato (Reserva Natural das Berlengas) and Silverio (Parque Natural da Ria Formosa). GPS loggers were financed by the EU INTERREG project FAME: The Future of the Atlantic Marine Environment (2009-1/089) and by LIFE + Berlenga (LIFE13 NAT/PT/000458). VHP, JGC and FRC acknowledge their Grants (SFRH/BPD/85024/2012; PD/BD/127991/2016 and SFRH/BPD/95372/2013, respectively) attributed by the Foundation for Science and Technology (FCT; Portugal) and the European Social Fund (POPH, EU). This study benefited from the strategic program of MARE, financed by FCT (MARE—UID/MAR/04292/2013). We thank the two reviewers for their useful comments on the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interests.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed and all necessary approvals have been obtained by the ‘Institute for Nature Conservation in Portugal (ICNF)’, which include the deployment of data loggers and collection of blood samples—Permits nos. 152/2011/CAPT, 101/2012/CAPT, 99/2013/CAPT, 203/2014/CAPT, 169/2015/CAPT, 133/2016/CAPT.
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