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Feeding ecology of coexisting Heermann’s gull Larus heermanni and elegant tern Thalasseus elegans chicks, based on stable isotope measurements

Alexei Elias-Valdez, Enriqueta Velarde, Alejandro Medina-Quej, José Manuel Castro-Perez, Joan Navarro, Rigoberto Rosas-Luis*

*Corresponding author:

ABSTRACT: Heermann’s gull and the elegant tern are considered “Vulnerable” by the International Union for Conservation of Nature's Red List of Threatened Species and Under Special Protection under the Mexican environmental law. Close to 95% of the world populations of both species breed on Isla Rasa (Gulf of California). As both species breed on the same island, competition for trophic resources can be expected to be high. Here, the feeding ecology of chicks of both species, as well as the contribution by prey species provided by adults was studied using δ13C and δ15N analyses in three consecutive breeding seasons (2017, 2018 and 2019). Results of the stable isotope analysis revealed that, although some segregation in the δ13C and δ15N values exist between both species, isotopic mixing model outputs indicated that fish was the most important prey group for both. However, at prey species level, it was found that the most important prey consumed by Heermann’s gulls were northern anchovy, Pacific sardine, and Pacific mackerel, followed by pelagic crab, whereas elegant terns consumed mainly northern anchovy. The trophic width estimated with the stable isotope values during 2017 and 2019 was higher for Heermann’s gull than for elegant tern. In contrast, for 2018, the trophic width of elegant tern was higher than for Heermann’s gull. In addition, both species showed a high degree of trophic width overlap during 2017 and 2019 (overlap probability 0.67 and 0.79, respectively). These results highlight the ability of Heermann’s gull to redirect its food intake to what might be the most accessible prey groups, a feeding strategy that allows the coexistence.