Barbara, the Nancy Birdwell who married in 1801 is one of our mystery folks. There are two possibilities;
1) She was from a yet to be found, parallel line of Birdwells who were not descendants of George Birdwell (c.1720 to 1781).Or ...
2) She was a daughter of George’s eldest known son, Robert Birdwell (c.1750 to 1815).
There are some “mysteries” out there on the earliest Birdwell members which could allow for the possibility that our George (c1720) had a brother or cousin in the same area during the late 1700’s. This undiscovered “theoretically possible” kin to George could have then had some children. However, while possible there is no known records of any Birdwell who can be positively identified as having not been George’s descendant. On the other hand, while also not proven, everyone found thus far in the early VA / TN records could possibly still trace back to George...
If that is the case and there was not a yet to be discovered, separate line of Birdwells, then George’s oldest named sons were; Robert (c.1750), George (1760), Ben (1765), Joseph (1767), Moses (1769), ... who was Nancy’s father ? You can exclude everyone but Robert.
George (1760). Kids named in 1816 will. No Nancy listed. Ben (1765). Kids named in pension filing. Daughter Nancy born 1807. Joseph (1767) Kids named in 1801 probate. No Nancy listed. Moses (1769). Eldest suspected child wasn’t born until 1791.
The number & identity of Robert Birdwell’s (c.1750) kids are very uncertain. Even his year of birth is uncertain. He may have been born well prior to 1750 and was probably already a father by the mid 1760’s. Note though that both of these points are unproven. All we know for sure is Robert was born before George (10-18-1760), and that Robert had at least one son.
The only proven child of Robert was “Joseph” named in the probate since he was still a minor in 1815. Otherwise you’ll find there are several competing lists of possible children for Robert. I have nine names down on my working draft of his possible kids. Your Nancy who married in 1801 is among those nine.
I have no evidence that Nancy Birdwell Hagan was Robert’s daughter. It is the absence of alternatives which leads me to suspect she was. Records show Robert was in east Tennessee from about 1787 until about 1812. So while unproven, his presence in the area in 1801 supports the possibility that he could be Nancy’s father. I would welcome any other data you have on Nancy and her family, perhaps we can discover more circumstantial evidence.