Originally posted by Swift1 Since you have clouds/overcast, the lighting will not be changing much.
I don't do a lot of bird photos but I would try this,
When you get to your location, set your camera to shoot RAW, set it to M mode.
Then, choose your ISO(I might start with 400) and f/stop, adjust shutter speed to +1 on your meter and take a test shot. Check the histogram, if there is blank space on the right side, lower your shutter speed, or open your aperture, or raise your ISO. Do this until your histogram is just barely clipping on the right side.
Then shoot away, keeping an eye on the histogram.
I'm pretty new to the World of Pentax having just picked up a K-30 a couple of weeks ago. But, one of the things that astonish me about this camera is the ability to produce REALLY good images in the higher ISO ranges. I'm assuming the K-5 can do the same. I shoot with the Sigma 170-500mm and I get some great results at ISOs ranging from 800 to 6400 and sometimes higher. Have you tried using these ranges?