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We are what we eat, plus some per mill: Using stable isotopes to estimate diet composition in Gyps vultures over space and time

Baino, Allan1; Hopcraft, Grant2; Kendall, Corinne3; Munishi, Linus4; Behdenna, Abdelkader5; Newton, Jason2

Published Sep 07, 2021 on Dryad. https://doi.org/10.5061/dryad.1ns1rn8qf

Data files

Sep 07, 2021 version files 1.55 MB
Sep 07, 2021 version files 1.55 MB

Abstract

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.