The Bird-type is an unused type in the first and second generations of Pokémon games.
In addition to the standard fifteen Pokémon types of Pokémon Red, Blue, and Yellow, and the seventeen types in Pokémon Gold, Silver and Crystal, there is another type hidden in the programming. This type is called Bird (not to be confused with the Flying type), and its index number is 06.
Its attacks are physical, or would be if any existed. No attack is of the Bird type and no Pokémon is of the Bird type. There are no types super-effective or not very effective against it, nor are any types weak or resistant to it.
Coincidentally, Missingno. and 'M (00) are partly Bird-type. However, 'Bird' never comes into play outside of Pokémon glitches or hacking.
This type appears in Pokémon Red, Blue, and Yellow, but also exists in Pokémon Gold, Silver and Crystal.