| Summary: | Darwin’s finches are a classic example of adaptive radiation involving differential use of dietary resources among sympatric species. Here, we apply stable isotope (δ<sup>13</sup>C, δ<sup>15</sup>N, and δ<sup>2</sup>H) analyses of feathers to examine ecological segregation among eight Darwin’s finch species in Santa Cruz Island, Galápagos collected from live birds and museum specimens (1962–2019). We found that δ<sup>13</sup>C values were higher for the granivorous and herbivorous foraging guilds, and lower for the insectivorous finches. Values of δ<sup>15</sup>N were similar among foraging guilds but values of δ<sup>2</sup>H were higher for insectivores, followed by granivores, and lowest for herbivores. The herbivorous guild generally occupied the largest isotopic standard ellipse areas for all isotopic combinations and the insectivorous guild the smallest. Values of δ<sup>2</sup>H provided better trophic discrimination than those of δ<sup>15</sup>N possibly due to confounding influences of agricultural inputs of nitrogen. Segregation among guilds was enhanced by portraying guilds in three-dimensional isotope (δ<sup>13</sup>C, δ<sup>15</sup>N, and δ<sup>2</sup>H) space. Values of δ<sup>13</sup>C and δ<sup>15</sup>N were higher for feathers of museum specimens than for live birds. We provide evidence that Darwin’s finches on Santa Cruz Island tend to be generalists with overlapping isotopic niches and suggest that dietary overlap may also be more considerable than previously thought.
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