Where would you find bird sounds, or where would you add an audio track?
It sounds like you are new to After Effects. Spending about 30 minutes in the Learn workspace to figure out the basic workflows and the basics of the UI will save you hours of fiddling. It would help if you also spent some time with the User Guide.
If you already have bird sound effects, import the audio files (WAV files are preferable to MP3), then drag them to the timeline. You can only put one source file on a layer. If you need 20 instances of a sound effect, you will need 20 layers.
If your video is longer than a few seconds, you should consider creating multiple compositions, rendering them, and then doing the final editing and audio mixing in Premiere Pro.
After Effects is not a video editing app, it is a motion graphics, animation, and visual effects application designed to create shots that cannot be created in an NLE.
Hi, Marilyn - If you are putting together a video using images and some sound effects, maybe a little music, I would recommend giving Premiere Rush a shot. Adding video and sound is just a drag-and-drop process.
As far as acquiring bird sound effects, give https://freesound.org/ a try. There are many sound effects libraries out there, but just be careful with the sites you visit.