Statistical analyses
Six linear mixed effects models were used in the R package
lme423 to analyze the effect of class, species,
individual, and tissue type on resulting isotope values. The first two
models? revealed a significant difference between birds and mammals,
thus additional models were run with mammals only and birds only to
clarify additional effects of the remaining variables onδ 13C and δ 15N values.
Tukey’s HSDs were used to elucidate differences among tissues for bothδ 13C and δ 15N values.
A conditional R-squared test was used to confirm the model fit.