Greg,
That makes sense. My intent on using the flash was precisely for these birds flying close by and NOT
"stadium" usage you see at concerts and sports events. These birds were flying low and sometimes few
feet from you. This one was backlit by Sun and it was about 6 feet (~2M away). Hence my excitement
though I admit I had no idea about distance calculations you had in your post. I was going by proximity I
observed on my outing couple of weeks ago.
What I cannot say for sure, is that given the high ISO, was the flash a factor at all. But then at 1/6400,
I could not say for sure it did not. Hence my post so that I can get better insight. I had a hypothesis in
mind and played it out and learned that the hypothesis did not hold water.

On to other experiments
and learning.
On the positive side, it was a mention in one of the posting (I can't remember who mentioned it, might have been Mark)
that don't ever try birds in flight at below 1/1600 as a rule of thumb was the piece of wisdom I needed.
As the saying goes, it you succeed, lead with it, if you fail, be guided by it.
As an aside, it was fun observing people wondering what the heck I was doing with flash firing on a nice bright day.
With best regards,
Sury