Stratis just feels really...empty without birds.
Wherever you are in Stratis, you're always hearing crickets chirping, or just ambient wind. I mean, not even the seagulls make any noise. About a third of Stratis is covered in forest, so you would expect there to be birds flying around and chirping, at least.
Hearing birds and/or seeing them fly overhead while walking through a forest would make the island seem more...habitable and alive...less desolate...and would be nice to listen to. Stratis' appearance is a lot more green and alive than it is in real life - so it deserves more creatures. Also, eagles or birds of prey would probably be lurking around Stratis, because of all the rabbits and stuff.
I also read another post about an idea of flocks of birds flying away when you start shooting in a forest, possibly giving away your position to any nearby enemy...That should probably be added too.
So hopefully all this stuff is coming in a future update...or the full game...:D