Museum specimens reveal ecological and morphological differences between North and South Island Kōkako (Passeriformes, Callaeidae)

Natural history collections offer exceptional opportunities to reconstruct long‐term ecological and morphological patterns, particularly for extinct or declining species. Here, we integrate stable isotope and geometric morphometric analyses of museum specimens to investigate trophic ecology and bill shape of the North Island (NI) Kōkako Callaeas wilsoni and the South Island (SI) Kōkako C. cinereus , the latter regarded as extinct. We measured δ 13 C and δ 15 N values and bill morphology from specimens collected between 1873 and 2005 (NI) and 1820 and 1911 (SI) to assess habitat use, diet, stable isotopic niche metrics and morphometrics over time. The δ 13 C data showed no temporal trend for either species, indicating stable habitat use and consistent association with C 3 ‐dominated forest environments despite extensive landscape modification in New Zealand. In contrast, δ 15 N values declined significantly in both species, revealing a long‐term shift towards a more herbivorous diet, probably caused by reduced invertebrate availability. NI Kōkako exhibited higher δ 15 N values than SI Kōkako, suggesting a slightly greater consumption of invertebrates, and δ 15 N values were significantly associated with bill morphology within NI Kōkako, in which individuals with higher and stouter bills consumed more invertebrates. Stable isotopic niche metrics indicated that SI Kōkako exhibited a broader stable isotopic niche than NI Kōkako, yet this apparent trophic flexibility was insufficient to prevent its population decline and extinction, suggesting the overriding influence of predation by introduced mammals for the population collapse. Our findings highlight the power of integrating stable isotopic and morphological approaches in museum specimens to reconstruct the ecology of extinct and threatened species to better inform conservation strategies for surviving populations.