[onirom]

A VSTi i have used several times for birds & animals is "sounds of nature" by Xoxos, maybe you could take a look at how it is done, my guess is some fast FM modulations.

To stay in topic i have synthesized some nature sounds (birds, waters) myself using this synthesizer and a simple two oscillators FM setup with massive pitch modulations for each oscillators, you can hear it on :

https://youtu.be/25o4Ru8kbpE?t=1111

And FM squeaking doors : https://youtu.be/25o4Ru8kbpE?t=1404

So my guess is that some FM with pitch modulation and many oscillators may be the way to go.

Percussion can be synthesized with convincing results through resonant models (simple resonant filters with initial noise), this is called modal synthesis, you can find filters parameters for some real instruments here : http://www.csounds.com/manual/html/MiscModalFreq.html