Ecological niche theory dictates that sympatric species cannot occupy the same ecological niche at the same time. Sympatric granivorous finch species in tropical savannas appear to contradict this theory by moving in mixed-species flocks and feeding together upon the same resources. Here, we explored this contradiction by tracking individuals from three finch species (Gouldian—Chloebia gouldiae, long-tailed—Poephila acuticauda and masked finch—P. personata) often seen feeding together, while simultaneously determining the dietary composition during periods of high and low resource abundance. Dietary composition was characterised using stable isotope ratios of δ13C and δ15N within the blood, and foraging areas were determined from continuous detection using passive VHF radio telemetry. Individuals were sampled early in the dry season when grass seeds were abundant, in the middle of the dry season and at the end when seeds were very scarce. The results showed plasticity in ecological niche overlap among the three finch species depending on grass seed availability within the landscape. In the early dry season, the species foraged together on the abundant grass seed. As the dry season progressed and grass seed availability declined, masked and long-tailed finches exhibited greater spatial partitioning, masked and Gouldian finches increased dietary partitioning, and long-tailed and Gouldian finches increased both dietary and spatial partitioning. Our study showed that sympatric granivorous finches utilise trophic and spatiotemporal partitioning to enable coexistence through periods of low resource availability in tropical savannas. Gouldian finches increased activity and ranging behaviour, while long-tailed and masked finches diversified their diet. Read the free Plain Language Summary for this article on the Journal blog.