The causal link suggested was because flying animals would need to be light, their digestive system evolved to be shorter than similar land based animals and al

The causal link suggested was because flying animals would need to be light, their digestive system evolved to be shorter than similar land based animals and also save the weight of the microbiome passengers(or it could be with shorter digestive time, no bacteria evolved to be symbiotic quick enough to be beneficial), so there is nothing like the symbiosis between microbiome and host animal that we humans and all other animals with backbones possess (phylosymbiosis). I'm curious how much they sampled non flying birds - it seems like a reasonable test for their hypothesis. There was some bacteria in flying animals guts, just not consistent in species like it is in animals that need phylosymbiosis or as much as non flying.