As in your other, similar, thread, you are asking questions to which there are no simple answers. The idea of a "flying bird" is so broad as to be virtually meaningless. What size is it, what distance is it at, at what speed is it flying, how is it lit, how much of the surrounding environment do you want to include, etc., etc., etc.??
Take this photo of a gannet in flight as an example:
It was taken with a Nikon D3s camera and a Sigma 150-500mm lens at 230mm, ISO was 800 and the exposure 1/1600 sec at F/7.1 and the camera to bird distance was probably about 15 metres with the image being cropped to about 25% of the full frame.
But I could just as easily have taken a similar photo with my Panasonic GX7 mirrorless CSC and the Panny 45-200mm lens (using the viewfinder, of course, not the stupid rear screen!)
At the end of the day, getting pics like that are about being in the right place at the right time, developing your fieldcraft skills and not worrying too much about your equipment. To a very large extent the type of camera is the least of your worries.