In reply to Greasy Prusiks:
Some sounds will be faked afterwards, but not all. There's usually a mixture of foley, archive and field recordings, and some field recordists will go to extraordinary lengths to capture sound - even building specific mics for certain jobs.
There's a brilliant radio doc about sound recordists here:
http://99percentinvisible.org/episode/the-sound-of-sports/
Deals with sport rather than nature docs, but a good insight into what techniques are used.
This is a great article too
http://www.theguardian.com/film/2015/jul/22/rain-is-sizzling-bacon-cars-lio...
Lots of sound recorded for documentaries and film can't actually be used, because it's not the sound we're expecting to hear, so sounds odd.
Post edited at 12:00