Intraguild niche partitioning in granivorous birds from the late past

Niche partitioning is a widespread ecological strategy within trophic guilds, ensuring the coexistence of sympatric species by reducing interspecific competition. Stable isotope analysis of carbon and nitrogen and isotopic niche metrics (width and overlap) are used as a proxy to investigate niche partitioning among species of a guild. In our study, we investigated if niche partitioning was an ecological mechanism contributing to the coexistence of granivorous birds from the same geographic region along time, employing an isotopic approach. We sampled and isotopically analyzed (δ¹³C and δ¹⁵N values) wing feathers from 58 specimens of granivorous birds collected between 1900 and 1966 in southeastern Brazil. We grouped birds according to the main habitat type used by them (forest: Cyanoloxia brissonii, Leptotila rufaxilla, and Leptotila verreauxi; and grassland: Columbina squammata and Sicalis flaveola). We used the Isotopic Richness (IRic) metric to measure the isotopic niche width and the Isotopic Similarity (ISim) and Isotopic Nestedness (INes) metrics to measure the niche overlap between groups and species. The results of low isotopic niche overlap suggest that common granivorous birds had a clear niche partitioning in southeastern Brazil in the late past, especially through foraging in distinct habitat types (forest and grassland; ISim ​= ​0.1, INes ​= ​0.2). Niche overlaps in almost all species-by-species comparisons were low (ISim ≤0.3, INes ≤0.4), except between C. brissonii and L. verreauxi (ISim ​= ​0.6, INes ​= ​1.0). These results suggest that these birds, belonging to the same trophic guild, coexisted through niche and resource partitioning. Despite being considered a plastic trophic guild, the isotopic niche partitioning among granivorous birds indicates that each of these species uses a set of specific resources and habitats. This raises an ecological concern about the homogenization of landscapes across the Neotropics, oversimplifying food resources and habitats to granivorous birds.