The extent to which the avian microbiome is shaped by host phylogeny
relative to other factors is largely unknown. In this study, we examine
microbial biodiversity across multiple body sites of 211 bird species
sampled in Malawi. Microbial community dissimilarity differed
significantly across body sites, which included blood, buccal cavity,
gizzard, intestine, cloaca, liver, and spleen. With these data, we
tested the hypothesis that the avian microbiota follow a Species-Area
Relationship by using a comparative phylogenetic method to examine the
correlation between microbiota richness and host weight. Using Pagel's
lambda, we confirmed that bird mass is significantly correlated with
host phylogeny but found that few microbial diversity metrics showed
such a correlation. Phylogenetic Generalized Least Squares identified a
significant but weak negative correlation between host weight and
microbial richness of the blood and a similarly significant but weak
positive correlation between the cloacal microbiota and host weight
among birds within the order Passeriformes. Taken together, these
results suggest that the avian microbiome does not follow a traditional
species-area relationship when phylogenetic relatedness is considered,
rather, microbial diversity is influenced by factors beyond host
phylogeny and size.