The Bird Dopant is a Dopant form assumed using the Bird Gaia Memory and an antagonist in episodes 17 and 18 of Kamen Rider W.
The Bird Memory had multiple users and could also be used by those without a Living Connector, though using it repeatedly has an addictive side effect.
Kamen Rider W
Saeko Sonozaki gave the Bird Memory to Akane Egusa, having selected her to test the limits of the new Gaia Memory.
After Akane ran away from home, her friend Touma Fujikawa came into possession of the Memory. Touma brought it to his friends Yuichi Kanamura and Yayoi Kubota. The three began using the Memory without a Living Connector to become the Bird Doapnt and eventually starting using it to terrorize people until they were stopped by Kamen Rider Double. After being defeated, the boys were taken to the hospital due to the negative side effects of using a Gaia Memory without a Living Connector.
Later, after suffering withdrawal, Akane attacked Philip and got back the Bird Memory. Upon using it again, Akane was consumed completely by the Gaia Memory and was able to assume the Bird Dopant's Strengthened Form. However, with assistance from the Nasca Dopant, Kamen Rider Double located the Bird Dopant's weak spot and defeated it using CycloneTrigger's Trigger Bat Shooting Maximum Drive to strike the Bird Memory.
Showa Rider x Heisei Rider: Kamen Rider Taisen feat. Super Sentai