SS and panning

Panning to follow a slow moving bird (Great Blue), 45 degrees in 1-1/2 second, tripod with remote, 300 mm set for f/6.3 and Ds3.

What SS should I be using?

Depends. Use a slow shutter to show motion, or a fast shutter to freeze motion.

Panning only compensates for the forward motion of the bird, not for its intrinsic motion, such wing flapping, head bobbing etc.

Define fast as I want to freeze motion.

Faster the better. These birds aren't as slow as they look. They are just large. You will need 1/800 or faster.
You really threw me with your question. You asked for SS and in your definition put 1 1/2 second.
Now I get what you are doing.

To freeze the wings of large birds (Great Blue Heron) in normal flight, 1/400s would normally work.

To freeze the wings of large birds in active flight or medium birds (gulls) in normal flight, 1/800 sec would normally work.

To freeze the wings of small songbirds, try 1/1600 sec.

To freeze the wings of hummingbirds, try 1/3200 sec.

Basically, go for as fast as the light will allow, and up the ISO if the bird is small and frantic.

Once you've mastered birds, try freezing the wings of dragonflies, bees, and mosquitoes., Now there's a challenge for you!