We are what we eat, plus some per mill: Using stable isotopes to
estimate diet composition in Gyps vultures over space and time.
1. Dietary studies in birds of prey involve direct observation and
examination of food remains at resting and nesting sites. Although these
methods accurately identify diet in raptors, they are time-consuming,
resource-intensive, and associated with biases from the feeding ecology
of raptors like Gyps vultures. Our study set out to estimate diet
composition in Gyps vultures informed by stable isotopes that provide a
good representation of assimilated diet from local systems.
hypothesized that differences in Gyps vulture diet composition is a
function of sampling location, and that these vultures move between
Serengeti National Park and Selous Game Reserve protected areas to
forage. We also, theorised that grazing ungulates are the principal
items in Gyps vulture diet.
3. Through combined linear and Bayesian
modelling, diet-derived from 13C in Gyps vultures consisted of grazing
herbivores across sites, with those in Serengeti National Park consuming
higher proportions of grazing herbivores (> 87%). 13C
differences in vulture feather subsets did not indicate shifts in
vulture diet and combined with blood 13C, vultures fed largely on
grazers for ~159 days before they were sampled.
Similarly, 15N values indicated Gyps vultures fed largely on
herbivores. 34S ratios separated where vultures fed when the two sites
were compared. 34S variation in vultures across sites resulted from
baseline differences in plant 34S values, though it is not possible to
match 34S to specific locations.
4. Our findings highlight the
relevance of repeated sampling that considers tissues with varying
isotopic turnover and emerging Bayesian techniques for dietary studies
using stable isotopes. Findings also suggested limited vulture movement
between the two local systems. However, more sampling coupled with
telemetry is required to fully comprehend this observation and its
implications to Gyps vulture ecology and conservation.