I have a feeling it's the simple fact that most bird species can fly and many of those species migrate incredible distances. This means (1) bird populations spr

I have a feeling it's the simple fact that most bird species can fly and many of those species migrate incredible distances. This means (1) bird populations spread out and (2) they face survival pressure across multiple ecosystems. Caveat emptor, that's just my layperson's "makes sense to me" hunch. No credentials or citations.