Morphological and stable isotope data from sympatric cormorant species in southern Chile

This dataset contains detailed morphological measurements and stable isotope values (δ¹³C and δ¹⁵N) for two sympatric cormorant species—Imperial Shag (Leucocarbo atriceps) and Red-legged Cormorant (Poikilocarbo gaimardi)—sampled at Pirén Islet, Reloncaví Sound, southern Chile. Data were collected during both breeding and non-breeding seasons to assess individual and population-level variation in trophic niche breadth and overlap.Morphological data include mass (kg), wing length (cm), bill length (cm) and width (cm), tail length (cm), and leg length (cm) for each captured individual.Isotopic data were obtained from three tissue types (feathers, plasma, and red blood cells) to provide temporal resolution of diet and foraging ecology. Isotope measurements were performed at the University of New Mexico's Centre for Stable Isotopes using an elemental analyser coupled to an isotope ratio mass spectrometer.Methodology:Cormorants were captured at night using light-attraction techniques and hand nets.Blood and feather samples were collected following non-lethal protocols and under ethical approval (Res. N° 7850/2021, Servicio Agrícola y Ganadero, Chile; CEC-Ulagos ORD. 72/2021).All individuals were measured, banded, and released after sampling.Isotope analysis was conducted according to standard protocols, with δ¹³C and δ¹⁵N values calculated relative to international standards.Use:This dataset enables analyses of seasonal and interspecific variation in isotopic niche width, individual specialisation, and foraging strategies. It is suitable for ecological, evolutionary, and conservation research on seabird trophic ecology and coexistence mechanisms.Ethical compliance:All data collection procedures were approved by relevant institutional and government authorities, ensuring adherence to animal welfare and legal requirements.For further methodological details, refer to the associated manuscript:"Individual and population variation in isotopic niche between two sympatric cormorant species."