The Bird type is a glitched type of Pokémon, possibly a beta Flying type. It currently does not apply to any natural Pokémon, but does appear in some glitched placeholder Pokémon like Missingno. Oddly enough, there have been some references over the years to it. For example, when Buena's Password is something like "Grass Pokémon", often an option will be "Bird Pokémon". This is likely a mistake on the developer's part because, as aforementioned, the Bird type does not exist in any non-glitch Pokémon. Do not save during this glitch or else it could corrupt your file.