Biodiversity plays a vital role in ecosystem functioning, so understanding how species coexist is a cornerstone of ecology. However, despite decades of research, our current knowledge is incomplete due to methodological limitations and sampling bias, particularly in the species-rich tropics. In this study, we combined bill and body morphological traits with stable isotopes in feathers to quantify niche differentiation among six co-occurring greenbul taxa, a diverse group of frugivorous and insectivorous passerines with remarkable similarities in body shape, in the lowland rainforests of Mt. Cameroon, West Central Africa. Our results showed that the greenbul’s niche space was primarily differentiated by variations in body morphology, with Yellow-lored Bristlebill Bleda notatus and Eastern Bearded Greenbul Criniger chloronotus occupying ecological niches distinct from the remaining four taxa, while bill morphology indicated substantial overlap between the taxa. In addition, isotopic composition of the feathers revealed a separation of Western Greenbul Arizelocichla tephrolaema from the other taxa. Our results show that the integration of morphological and isotopic data can provide robust estimates of niche overlaps, providing evidence for the differentiation of ecological roles. This highlights the importance of integrating variable traits to improve our understanding of how animals exploit the multidimensional niche space that enables their coexistence.