I noticed they say "bubo" is masculine AND feminine. (weird!)
So, if it's right, and not contextual, I guess it should be rather:
Estne bubo ebrius?
Or: Estne bubo ebria?
Edit: it's an hapax!
It means that the feminine nature for "bubo" has been only attested once. It's written in Lewis & Short:
" feminine only once Vergilius".
But the dictionary mention both gender, probably because it was found in Vergilius...
So, we can still use "bubo" as a feminine noun if we want, it's not grammatically wrong, only very unusual.
vesper is can be found as either a second declension -er or a third declension. Wiktionary says that "the second declension forms prevailed except for the ablative".
vesperi here is an adverb that comes from an ablative form. vespere being another variation that exists. Based on section I. Lit. B. 1. at:http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0059:entry=vesper