Actually it doesn't mater - 600mm (effective - I should better ask "what camera") and speed 1/320 are what they are! My first series (the orange-yellow dragons) are at effective 750mm (crop camera, the actual focal length is 500mm) at speed 1/8000. You can't make the clear blue sky blurred but in two of the photos you see greenish background low in that photos - it's not a grass, it is small trees. Blur is coming from the panning and distance. In the second series it's again effective 750mm at speed 1/2000 (catching them at relatively slow motion) - so high to get the wings at least slightly blurred (at these photos the Dragons look like motionless and I don't like it). Panning on very unstable tripod because of the ground, VR on, Markins Q20 ball-head (I don't use a gumball: heavy for hiking).Hi ISv.
My guess would be that it is from his 300 f/2.8 + 2x converter days.