This game is in Early Access on Steam

and is not being updated on itch right now, because it heavily depends on Steam's multiplayer features. You can still play the itch version and take it as a sort of a demo.
https://store.steampowered.com/app/3682550/Birb_Game/

Flight controls

Fly - Left and Right mouse buttons or Left and Right trigger (both at the same time)  

Glide - No mouse buttons or Triggers  

Steering - Left or Right mouse button or joystick (you can steer while gliding, always rotating to the opposite side than the Button/Trigger you're holding)  

Dive - Q or Left/Right shoulder button