From the Super Mario Wiki
Skeleton Birds (スケルトンバード, Sukeruton Bādo) are enemies in the game Wario Land 4. They appear in the levels Sapphire Passage and Golden Passage. As their name implies, they are flying skeletal birds that cannot be normally destroyed by Wario. Skeleton Birds wear a green hood, a red bandanna and have two wings. They attack by spitting green clumps of goo, which do not actually hurt Wario, but turn him into Zombie Wario. While Wario is a zombie, he can attack Skeleton Birds; after hitting them once, they will crumble into a head which Zombie Wario or his normal self can defeat.
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