Results
The analysis of the linear mixed-effects model that contained both mammals and birds highlighted significant variability between the two classes for both δ 13C andδ 15N values. Further analyses were conducted on mammal and bird data independently.
The linear mixed effects model for mammals showed a difference of -1.93 ‰ (SE = 0.08) between the δ 13C values of homogenized whole body (CWB) and the keratin outer covering (CK) Fig. 1a), but the other pairwise relationships were not significant (Table 1). These findings imply that the presence or absence of keratin outer covering did not significantly alter the isotopic value of the homogenized mass in either birds or mammals (Table 1). CK and CWB did not have a significant relationship in birds.
The linear mixed effects model for birds showed a difference of -0.32 ‰ between NWB and NK (SE=0.05) (Fig. 1 b) birds, but not mammals, and the other pairwise relationships were not significant (Table 1).