An ecological niche has been defined as an n-dimensional hypervolume formed by conditions and resources that species need to survive, grow and reproduce. In practice, such niche dimensions are measurable and describe how species share resources, which has been thought to be a crucial mechanism for coexistence and a major driver of broad biodiversity patterns. Here, we investigate resource partitioning and trophic interactions of three sympatric, phylogenetically related and morphologically similar species of thrushes (Turdus spp.). Based on one year of data collected in southern Brazil, we investigated niche partitioning using three approaches: diet and trophic niche assessed by faecal analysis, diet and niche estimated by stable isotopes in blood and mixing models, and bipartite network analysis derived from direct diet and mixing model outputs. Approaches revealed that the three sympatric thrushes are generalists that feed on similar diets, demonstrating high niche overlap. Fruits from C3 plants were one of the most important food items in their networks, with wide links connecting the three thrush species. Turdus amaurochalinus and T. albicollis had the greatest trophic and isotopic niche overlap, with 90% and 20% overlap, respectively. There was partitioning of key resources between these two species, with a shared preference for fig tree fruits - Ficus cestrifolia (T. amaurochalinus PSIRI% = 11.3 and T. albicollis = 11.5), which was not present in the diet of T. rufiventris. Results added a new approach to the network analysis based on values from the stable isotope mixing models, allowing comparisons between traditional dietary analysis and diet inferred by isotopic mixing models, which reflects food items effectively assimilated in consumer tissues. Both are visualized in bipartite networks and show food-consumers link strengths. This approach could be useful to other studies using stable isotopes coupled to network analysis, particularly useful in sympatric species with similar niches.

Birds were captured with 6 mist nets, 36 mm mesh size (12 m × 2.6 m), settled in locations adjacent or inside forest patches, in shadow areas, where intense transit of birds of interest had been observed. Nets were open 30 min before sunrise and closed 30 min after sunset. Captured birds were identified and placed in cotton bags for 15 min for the collection of faeces and regurgitated material.

Faecal samples were individually stored in vials with 70% ethanol, in the laboratory faeces were screened in a stereomicroscope, identified at the lowest taxonomic level possible using identification guides and consultations with experts. Food items were first counted individually, and then, their reconstituted volume was estimated based on the average volume of items of similar size in a reference collection. To quantify the items present in the diet, the volumetric method was used following Hellawell & Abel (1971) that was modified by Bastos et al. (2013). With a millimetric paper under a Petri dish, food items were compacted with glass slides, forming blocks of 1 mm³.

A total of 100–200 µl of blood from the brachial or tarsal vein of birds was collected with a syringe and needle (4 mm), stored in plastic vials and frozen. Blood samples were lyophilized for 8 h, homogenized, weighed and placed in sterilized tin capsules (5 × 9 mm). Samples were analysed in isotope ratio mass spectrometer at the Stable Isotope Center at the University of New Mexico (UNM) in the USA. To determine the isotopic values of consumers, the isotopic ratio (R) of each element in the samples (¹³C/¹²C and ¹⁵N/¹⁴N), represented in delta notation (δ) and expressed in parts per thousand (‰), was obtained. Atmospheric air and Vienna Pee Dee belemnite (VPDB) were the international standards for N and C, respectively. Isotopic reference materials were interspersed with samples for drift calibration, and the precision of the measurements was 0.08‰ and 0.03‰ for δ¹⁵N and δ¹³C, respectively.