Assessing changes in species trophic ecology over a large timescale is essential not only to measure how anthropogenic impacts modify ecological processes but also to measure the effectiveness of wildlife conservation actions. The North Island (NI) and South Island (SI) Saddlebacks, or tīeke, Philesturnus rufusater and P. carunculatus, respectively, are two of the species undergoing the most effective conservation actions in Aotearoa New Zealand after large-scale management of introduced mammals and successful translocation of individuals. Here we used stable isotope analysis of carbon and nitrogen to determine isotopic niche metrics and assess whether the trophic ecologies of NI and SI saddlebacks differ over time. We sampled contour feathers from 33 NI saddleback (spanning 1880–2011) and 36 SI saddleback (spanning 1826–2012) specimens, dividing them into three temporal groups for analysis, i.e. historical, bottleneck, and post-translocation periods. The isotopic niche widths of both saddleback species differed significantly between the temporal groups, with different trends for each species. The NI saddleback showed an expansion in the post-translocation period niche width, indicating this species was probably on the verge of complete collapse after the intensification of anthropogenic impacts in New Zealand. In contrast, the SI saddleback showed a contraction in the post-translocation period niche width, which may reflect a less diverse habitat in terms of food resources. This demonstrates that conservation measures, although successful in terms of population re-establishment, could not completely restore the historical trophic niche of this endangered species. Additionally, SI saddlebacks had higher stable isotope values than NI saddlebacks, suggesting that the two species differ in their trophic ecology. We demonstrated that ecological metrics of successful conservation actions can also be measured through stable isotope analysis of museum specimens, in addition to providing information that supplements current knowledge of the species.
