This is the 1890th most frequent German word.
vogel
Bird.
Here, 'Vogel' is used as a noun referring to an actual bird.
Der Vogel fliegt hoch am Himmel.
The bird flies high in the sky.
In this idiomatic expression, 'einen Vogel haben' means 'to be crazy'.
Er hat einen Vogel, wenn er so handelt.
He is crazy when he acts like that.
Again, 'Vogel' is used as a noun referring to an actual bird.