For example birds:
https://freesound.org/search/?q=birdsongs
But! You guys must respect the terms of usage and give credits.
I suggested to one there, to put name on loading screen....... AND add a readme.txt to the final course that describes who and where you got the sound from.
And it's kinda nice to send the creator a message in advance to let him/her know what you gonna use it for.