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.