From ARMS Institute, the ARMS Wiki
Thunderbird is an ARMS weapon with the Electric attribute. It consists of two parts: a bird and its nest. When a fighter punches with the Thunderbird, the bird flies away from its nest in the direction of the punch, then returns to it.
"A bird in the hand is worth two in an opponent's face, especially if they're electrically charged and ready to attack from any direction!" —Official Description
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