The Pearl Bird is a strange, green bird with small wings and a mouth that can expand to a large size. It is only seen in two Puzzle Rooms throughout Wario Land 4, including one in the 40 Below Fridge and another in Crescent Moon Village. It doesn't harm Wario in any way; in fact, it actually helps him by spitting out glass spheres which Wario can catch and throw to solve certain puzzles to earn treasures.
In Super Smash Flash
Pearl Birds belong to the many aerial enemies in Super Smash Flash, alongside Flying Goombas, Buzzers, Beezleys, and Mayu Birds. They are exclusive to the aforementioned Floating Islands, where they follow the player around where contact with them results in the character taking damage and getting pushed back a bit. Though they can be persistent if a swarm surrounds the player, a single attack is enough to take them out.
The Pearl Bird is the only enemy in the Super Smash Flash series to not appear as an actual enemy in its original games, namely Wario Land 4.