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, these creatures are flying skeletal birds that can't be normally destroyed by Wario. Skeleton Birds wear a green hood, a red bandana and have two wings. They attack by spitting green clumps of goo, which does not actually hurt Wario, but turns him into his zombie form. While Wario is a zombie, he can attack a Skeleton Bird; after hitting it once, it will crumble into a head which Zombie Wario or his normal self can defeat.
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