**4. How to study owl diets**

Documenting prey capture by owls in the wild is exceptional unless a nest is found—although the specific identity of prey provisioned to nestlings cannot always be determined. Therefore, the study of pellets is pivotal for understanding species' diets given the ease of analyzing them and the number of samples that can be obtained with relatively little field effort, once roosting and feeding sites are located.

Following known procedures in pellet studies [9], we suggest following these steps to maximize data collection: 1) Remain at the site of pellet collection until species identity can be confirmed. Alternatively, camera traps can be set up for species documentation and identification. 2) Collect pellets individually and also all pellet debris. 3) Georeference the collection site and make a description of the area. 4) Air dry each pellet for at least 3 days, then measure its maximum length and width with a caliper and weigh it. 5) Soak the pellets individually in water and disaggregate them up to separate bones and other prey remains. From our experience in Ecuador, only skulls, jaws, beaks, mandibles, and elytra are useful for identification purposes. 6) Compare your voucher specimens directly with museum specimens. 7) Use the presence of unique structures, skulls, and pairs of mandibles or elytra to estimate the number of prey per pellet as the minimum number of individuals (MNI). 8) Calculate the percentage of occurrence as the MNI of each species by the total number of individuals of all species. 9) Calculate the biomass consumption as the mean body mass (in g) of each species multiplied by its MNI.

To calculate dietary niche breadth, we suggest standardized Levin's index [52], which varies from 0 (narrow trophic niche, maximum prey selectivity) to 1 (wide niche, minimum selectivity). Thus, when the values are less than 0.6, the organism is considered a specialist, since it uses a low number of resources and has a preference for certain foods [53, 54]. To determine owls' foraging strategy, it is necessary to study potential prey richness and abundance in the study area. Strategies could be opportunistic when it ingests the prey in the same relative abundances of its environment, or selective, when it ingests some or all of the prey in different proportions to those present in the hunting area [54]. Degree of dietary overlap between areas or seasons can be analyzed using Pianka's dietary niche overlap index or through a Chi-square test (χ<sup>2</sup> ) to check for differences in the diet composition between sites or seasons.
