I have been fortunate to be involved with recording quite a bit of Foley for nature documentaries over the last little while.
There are a couple of great articles from about 10 years ago that I often refer to in sound design classes that discuss the sound of nature documentaries. I won’t spoil the story, but if you are interested, you can read them here:
http://www.nealromanek.com/sounds-of-nature-part-i/
http://www.nealromanek.com/sounds-of-nature-part-ii/
There is also a more recent radio documentary discussing some interesting examples from nature documentaries of the past, and the stretching of the truth factor:
http://99percentinvisible.org/episode/sounds-natural/