I have tried it in P, S, and A. First using the suggested settings given on this thread where A is set, then I tried with S and finally P.
With S, after some fair amount of trial and error, I figure out what speed depending on the time of day, light availability, subject type that is to be captured (meaning wether the bird is constantly moving or stationary)...etc etc that I can almost dial up a speed setting and then fine tune. If subject is out in the open with very bright conditions will reccommend you give it a go.
With P, very time consuming, actually twice the amount of time and fiddling to get it right.
With A, very little camera adjustments and fine tuning to do.
As with everything in life, one can only get better and faster with learning and practice. I do get very good results with the camera set to both S and A but must admit that I have more quality pics with A where the subject is constantly hopping around in trees and underbush and time is in seconds of inactivity for a shot. If you are like I am, new to digiscoping and because of the limitations of the ISO settings with regards to noise in PS cameras then I would suggest the "A"settings reccommended in here as your starting point.