Thanks for the comments folks.
Artie: I took photos during a number of drums as the sun was rising and for a short period thereafter before the light quickly deteriorated due to the mix of sun and shadows. A drum typically lasts for about 7-8 seconds and is repeated every few minutes. These grouse have a remarkable ability to hold their head and body essentially motionless while rapidly beating their wings - this accounts for the sharpness of the head and belly - the breast feathers are blurred because they are being blown around by the wingbeats. Having said this, not all my shots at these rather low shutter speeds were sharp on the eye - I would attribute that to operator error more than to movement of the bird.

Near the end of the drum as wing speed reaches its peak, the bird will lean forward slightly and this minor movement can also contribute to soft heads at low ss. As the light (and ss) increased, the proportion of sharp heads increased. At ss of 1/20 - 1/60 the wing position, though blurred, will be different in each frame and it becomes a matter of choosing images with pleasing blurs.