Originally posted by mysterick I am using the 1.4 RC with the HD 55-300.
You lose one stop of light with the TC. This means that even wide open at 300mm the effective aperture would be f8. In less than ideal conditions, this will affect focus accuracy. These slow zooms are not really suitable for use with a TC in other than very good light IMO.
Like other posters I find single point AF (whether centre point or select) more accurate (with the proviso that I haven't used the K-3iii). But where the bird is fairly small in the frame, and you are aiming for the head/eye, you are trying to target a very small area. It's just inevitable that you will miss focus on a lot of shots.
I agree with what @FozzFoster and @Ramseybuckeye have said. Personally with the KP I use TAv and let the ISO float, but generally only to 3200 (6400 if necessary). 1/640th or 1/800th is plenty fast enough, but if light doesn't allow that I'd be prepared to drop the shutter (to 1/80th if necessary). Sometimes shooting a burst at a slow speed will get one or two reasonable shots, but you might want to try fill flash instead when the light isn't enough. This will very often give better colour and feather detail. Even the excellent accelerator unit on the KP can't perform miracles - higher ISO does produce more muddy images when the light is insufficient.
Any way to get closer to the feeder? That would make a lot of difference.