The Bird type is a glitched type of Pokemon-possibly a beta Flying type. It currently does not apply to any natural Pokemon, 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 Pokemon", often an option will be "Bird Pokemon". This is likely a mistake on the developer's part because, as aforementioned, the Bird type does not exist in any non-glitch Pokemon.
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