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

Abstract

The Heermann’s gull and the elegant tern are considered Near Threatened by the IUCN Red List of Threatened Species and under special protection under 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 stable carbon (δ13C) and nitrogen (δ15N) isotope analysis in 3 consecutive breeding seasons (2017, 2018, and 2019). Results of the stable isotope analysis revealed that although some segregation in the δ13C and δ15N values exists between both species, isotopic mixing model outputs indicated that fishes were the most important prey group for both. However, at the prey species level, 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 the Heermann’s gull than for the elegant tern. In contrast, for 2018, the trophic width of the elegant tern was higher than that of the 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 the Heermann’s gull to redirect its food intake to what might be the most accessible prey groups, a feeding strategy that allows the coexistence.