I only got out the 500 and tripod to catch him perching on that bale of hay- but since I had it out I thought I might get lucky in the vineyard. And yes, your 600mm with the D3s would have done a better job, escpecially with a TC-17E. Even at ISO 320 the D300's DR is really stretched here.
Love #3 In California hawks are all over in the vineyards...the food source for them there is huge...rabbits, ground squirrels, mice etc. I saw owls only a couple times but that was very early in the morning in the headlights of my car.
I've read the crushing power of Snow Owls is in the hundreds of pounds ... those claws have some amazing strength.