Interpreting past trophic ecology of a threatened species, kea (Nestor notabilis), from museum specimens

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.