Bird-Masked Anbu Member

He is an Anbu Captain from Konohagakure.

Background
He and another Anbu accompanied the Third Hokage in infiltrating a secret bunker with the intention to find out who was behind the disappearances of shinobi from their village. After they disarmed the traps, they confronted the person behind the acts and discovered it was Orochimaru. Having been caught, the Sannin attacked the Anbu and Hiruzen, giving him an opportunity to escape and flee the Land of Fire.

Appearance
He is dressed in a white cloak and black trousers with bandages tied around his right leg and black sandals. He has brown hair that sticks out his right side in a long bang. His mask varies between the manga and the anime. In the manga his mask has a triangular shape pointing down from the top and a short line running up from the right eye hole. In the anime these marking are replaced by four yellow lines on either side running in towards the centre of the face. Both masks however retain the bird-like motif.

Abilities
He is presumably a powerful shinobi, being an Anbu. He showed proficiency in Wind Release techniques, being able to use the Wind Release: Wind Cutter Technique effectively to attack one of Pain's summons.

Pain's Assault
During Pain's invasion this Anbu captain worked in a three man team with two other captains in order to battle Pain's giant multi-headed dog summon. This battle is expanded on in the anime as they use their "Formation B" in order to halt the beast's advance and allow Shizune's group time to escape.

Trivia

 * In episode 50, a similar looking Anbu appears alongside his partner and other Anbu.

ANBU con Máscara de Pájaro Anbu au Masque d'Oiseau