It is a species very similar to the other ground finches, but with a body and bill size intermediate between the Small Ground Finch (Geospiza fuliginosa) and the Large Ground Finch (Geospiza magnirostris). Adult males are black, females and juveniles are brownish with spots on the back, and the ventral side is grayish with streaks, generally less pronounced than in the Small Ground Finch. It is one of the species that generates the most problems when identifying it, as it has a wide individual variation, to the point of overlapping with the other species. Basically, to differentiate it from the Small Ground Finch and Large Ground Finch, the bill is the key, as the size impression in the field may fail, it is not as obvious. It has a bill notably more robust than the Small Ground Finch, especially at the base. Regarding the comparison with the Large Ground Finch, the Medium Ground Finch has a noticeably separated culmen from its forehead, while in the Large Ground Finch, the slope of the culmen continues into the forehead without interruption. Additionally, hybridization with the Small Cactus Finch (Geospiza scandens) has been documented (Markert et al., 2004; De Roy, 2022). Compared to this latter species, the body size is equivalent. Intermediate molt patterns in immature males could produce individuals with caps, which could resemble and even confuse with tree finches. Monotypic.

A female of this species is the progenitor of the new "Big Bird" lineage (Lamichhaney et al., 2017) from Daphne Mayor, the result of hybridization with a male of the Española Ground Finch (Geospiza conirostris).

Geographic Distribution: All Galápagos islands excluding Darwin, Wolf, Genovesa, and Española (BirdLife International, 2023). Declining in Floreana (Kleindorfer et al., 2022).

Description extracted from: La Grotteria (2023).

REFERENCES

La Grotteria, J. 2023. Identificación, comentarios y registros personales de pinzones de Darwin. Referencia Orientativa.

 See related literature