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The diet patterns and trophic relationships are poorly understood for most tropical owl species. We used stable isotopes of carbon (d13C) and nitrogen (d15N) in 24 feather samples of the rare, endemic, and 'Near Threatened' Bearded Screech-owl Megascops barbarus to determine the trophic level of their prey and evaluate whether diet patterns vary (1) among individuals, (2) spatially along the species's range in the highlands of Chiapas, Mexico, and (3) temporally during the short- and long-term. Our results indicated that there was diet variation among individuals during the period of feather growth and there was a high positive correlation between stable isotopes in body and rectrices. The stable isotopes showed significant temporal differences in d15N signature values, but not in d13C values, with no obviously interpretable temporal pattern. Spatially, values of d13C and d15N did not vary across all nine sampled locations. The observed lower d13C values suggested that this owl lives in humid forests. More long-term studies and spatial dietary and prey analysis will be necessary to increase our understanding of how habitat conditions determine the distribution, abundance and quality of food for the Bearded Screech-owl Inglés

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