You might want to try a couple of different things here
1) monopod, more for your tired arms than absolute stability
2) if SR is working take the lens down to F8 and slow the shutter to about 1/250. more DOF may be better than fast shutter.
3) set shutter to 1/150 and stop down to get about -3 stops on exposure, then use a flash. FLash freezes better than anything else
If you want to see flash on a 500mm equivelent lens, see the link below. SMC 300F4 plus 1.7x AF TC
https://www.pentaxforums.com/forums/106132-post4.html
I will look for a heron. I know I have a bunch done with 300mm And a flash.