Phylogenetic Generalized Least Squares
Samples analyzed by sample type and each alpha diversity measurement (observed, Shannon, and Simpson) from all samples from the same species were averaged together to get a single set of measurements per host species. In the full dataset, the non-phylogenetically controlled regression found a single test to be marginally significant (cloaca, Simpson, p =0.048); all other tests had p >0.05 (Table 1, Supplemental Fig. S2). Using PGLS on the full dataset, all three alpha diversity metrics for the blood microbiota were significantly associated with bird mass (p <0.001); all other comparisons were not significant (p >0.17). When looking at the Passeriformes-only dataset, the non-phylogenetically controlled regression found a single test to be marginally significant (cloaca, Shannon, p =0.022); all other tests hadp >0.05 (Table 1, Supplemental Fig. S3). Using PGLS on the Passeriformes-only dataset, all three alpha diversity metrics for the cloacal microbiota were significantly associated with bird mass (p <0.05). Two of the gizzard microbiota tests were also significant (p <0.05) but we note that this dataset has extremely low sample size (N=9 individuals from 8 species). All other comparisons were not significant.