We currently have the general avian, which for a long time has been described (in terms of the tag wiki excerpt) as
For questions about birds and birdlike creatures.
After a recent edit by yours truly, it is now described as
For questions about birds and birdlike creatures. Does not include machines designed for flying (consider [airships], [aircraft], [spaceships]). Consider also [flight].
However, there is currently no tag birds
I propose that
birds is established as a tag synonym, with the canonical tag being
birds is more discoverable, but
avian is more generic.
See also discussion on the factory floor, beginning here (link to the chat transcript).