Reason: Bird is the metatag for all the various birds on the site, but there's another metatag over Bird. That's Avian. There's almost no difference between Bird and Avian save that Avian includes gryphon while Bird doesn't. A search for Avian -gryphon should return all of the Bird results (and more, because the bird tag is far more often missed than the avian tag is). Since its effects can be replicated with a two-tag search, it seems to me that the bird tag is just bloat that should just go away.
Updated by Halite