With the P900 and bif pics you need good light first of all. Then like with any detailed bird picture with any camera, be as close to the subject as posible. A spec in the sky is not really a bif pic.
Camera setting as follow:
I save all in User settings so that if I see a bif opportunity I quickly dial to "U" and all settings are in place, including the predetermined focal length. When dialing back to your previous program, may it be "S" or "P" or whatever, it will go back to the settings you were using before dialing to "U".
ISO - 100-400 Auto range (To increase shutter speed.)
Shutter speed and aperture will be set in camera if you do your initial settings in "P" and then "Save user settings".
Metering - Center-weighted
AF area mode - Target finding AF.
Autofocus mode - Full-time AF
Focal length - 600mm eq.
Other settings like Whitebalance according to light conditions.
At first don't try too long focal length, it is not easy to keep a flying bird in the viewfinder. Start with say 400mm and move up as you get more experience. Pan with your body and not with your hands. First try bigger slow flying birds and then smaller one. Keep on trying and practice, practice, practice.
Even professional photographers don't get it right every time and delete a lot of flops. Keep on taking pictures and keep the better one's.