Forage fish to growing chicks: shared food resources between two closely related tern species

Abstract

The assessment of the relevance of forage fsh in seabird diet and how diferent species partition food resources is fundamental to understand predator–prey relationships. We assessed the importance of forage fsh in the diet of Royal (Thalasseus maximus maximus) and Cayenne (T. sandvicensis eurygnathus) terns and their partitioning of food resources during the early and late chick stages of 2013 and 2014 at a colony in Argentina. Direct observation of prey deliveries during chick provisioning showed that diet composition of Royal and Cayenne terns comprised at least 16 and 9 prey species, respectively. In both tern species, Argentine Anchovy (Engraulis anchoita) and fve species of silversides (Odontesthes spp.) were the main prey fed to chicks (over 90% contribution, anchovy and silversides pooled). Both tern species fed their chicks with similar prey species, but Royal terns delivered, in general, larger prey than Cayenne terns. Based on carbon and nitrogen isotopic values from chick whole blood samples, Bayesian mixing models showed that anchovies and silversides contributed similarly to the diet of both tern species’ chicks. Both conventional and stable isotope methods showed a high overlap in their trophic niches. Prey sizes delivered to chicks were larger in the late chick stage and the second study season. Wind speed did not have a signifcant efect on the frequency of the diferent prey species and sizes delivered to chicks by both tern species. As anchovies and silversides are fshery targets, tern trophic requirements should be considered when planning future fsheries development and management.