This is great, guys, thanks for the list! I sampled most of the ones posted.
I have to second the American Birding Podcast, probably the best produced and most focused one I listened to. Great conversations on there and cool notables from eBird. Nate Swick is a phenomenal host.
Not sure if anyone mentioned RadioLab. Kind of goes without saying, but they have done many fascinating nature-science episodes, and they are the gold standard of audio production and narrative journalism.
I also stumbled on The Music of Nature Podcast, but recordist Lang Elliott. Each episode is an audio postcard from his travels to birding hotspots throughout North America, with high-quality binaural (stereo) recordings to put listeners in the soundscape with some narrative between scenes. 10-30 mins long each. He posted several episodes from 1/26/17 to 12/18/18. Listened to the first one and I’ll definitely do them all.
This is tangentially nature related, but the show Sandra is an audio drama that comically features birds.
The Dirtbag Diaries. Not a nature podcast, per se, but an adventure podcast with a high view of nature and conservation/access activism.
Outside/In by New Hampshire Public Radio.
HumaNature by Wyoming Public Media.
Threshold. About bison.
Mountain and Prairie. Interviews with interesting people in the West.
Outlandish. Part of the USFS Your Forests Your Future project.