When ecosystems are under severe pressure or environments change, trophic position and intraspecific niche width may decrease or narrow, signalling that conservation action is required. In New Zealand, alpine and sub-alpine ecosystems have been extensively modified through farming since 19th century European settlement, with consequences for indigenous species such as the kea (Nestor notabilis). We investigated feather stable isotope values in the kea and predicted a lower trophic position in modern kea populations, to reflect reduced lowland habitat and a mixed diet with more plant material. We predicted that size and sex would influence trophic values in this sexually dimorphic species, with larger birds more likely to have a high protein diet. We examined potential dietary changes in 68 museum collected kea from 1880s to 2000s, first recording accession details including provenance and sex, and measuring culmen length. We used bulk carbon and nitrogen stable isotopes analyses (BSIA) of feathers and a further feather subset using compound-specific stable isotopes analyses of amino acids (CSIA-AA) to obtain isotopic values and estimate trophic position. BSIA showed δ15N values in kea feathers declined through time, and could indicate that early century kea were highly omnivorous, with δ15N values on average higher than in modern kea. Variance in δ15N values was greater after 1950, driven by a few individuals. Few differences between males and females were evident, although females in the south region had lower δ15N values. There was a tendency for large male birds to have higher trophic values, perhaps reflecting dominant male bird behaviour noted in historical records. Nonetheless, CSIA-AA performed on a subset of the data suggested that variation in BSIA is likely due to baseline changes rather than relative trophic position which may be more homogenous than these data indicate. Although there was more variability in modern kea, we suggest caution in interpretation. Stable isotope data, particularly CSIA-AA, from museum specimens can reveal potential change in ecological networks, as well as sexually dimorphic feeding patterns within species. The data can reveal temporal and regional variation in species trophic position and changes in ecosystem integrity to inform conservation decision-making.

We examined kea specimens and skins from 5 natural history collections, at Auckland Museum, New Zealand; Te Papa National Museum, NZ; Canterbury Museum, NZ; and Otago Museum, NZ; and Naturhistorisches Museum Wien (the Vienna Natural History Museum), Austria. We recorded all accession details for kea where these were available, including collection location, year of collection, and sex if previously determined. Stable isotope analysis was conducted from feathers removed from the upper middle back between the shoulder blades on each bird skin, and were washed in 2:1 chloroform:methanol solution for 24 h, agitated, rinsed and air dried in a fume cupboard for 48 h, before the top 1cm of the feather vane was finely clipped and weighed into tin capsules for bulk nitrogen and carbon isotope analysis. Further methods are reported in the paper.

Missing values do occur in this dataset; not all information required was available for all individuals. See the attached ReadMe file.