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 that stem from the feeding
ecology of raptors like Gyps vultures (Gyps
africanus and Gyps rueppelli). Our study set out to
estimate diet composition in Gyps vultures informed by
stable isotopes that provide a good representation of assimilated diet
from carrion resources in local systems.  2. We 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 to forage. We also,
theorized that grazing ungulates are the principal items
in Gyps vulture diet.  3. Through a combination
of linear and Bayesian models, diet derived from d13C
in Gyps vultures consisted of grazing herbivores across
study areas, with those in Serengeti National Park consuming higher
proportions of grazing herbivores (> 87%). d13C
differences in vulture feather subsets per site did not indicate vulture
diet change, and in combination with blood d13C, vultures fed
largely on grazers for ~159 days before they were sampled in both sites.
Similarly, d15N values implied
that Gyps vultures fed largely on herbivores across
space and time. d34S ratios separated prey source for
vultures between the two sites. d34S variation in
vultures across sites resulted from differences in baseline
(plant) d34S values, though it is not possible to
match d34S 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.