If you found a will, leaving property to Mary Birdsong from a Mr Armistead, that would seem pretty solid - but I assume the wills you found weren't so helpful. I agree that online genealogies are useful only as a starting point to search.
I only pursued this a little bit - probably 12-15 years ago. I found in my notes for Mary Armistead that I had found land abstracts on Anthony, William & John Armistead of Elizabeth City Co VA
Unfortunately, I don't know where those abstracts are and I assume they didn't give a connection to Mary - but those names could be a lead.