Since the Virgil's passage in question happens to start with 'Tum' I have to contribute something...
(This is not an answer but I see no way of putting this as a comment due to formatting issues.)
There appears to be no clarity on the issue, so the chances of getting a clear, authoritative answer are actually slim.
"The Royal Grammar, Commonly Called ...
The relevant passage is this one, from Aeneid IV.462-3:
sōlaque culminibus fērālī carmine būbo
saepe quer' et longās in flētum dūcere vōcēs
And the lone owl on the rooftops would cry out its mournful song, drawing out its long calls into an elegy.
I can see a few possible reasons why Vergil chose to make this particular būbo a feminine one: