You use the effect of Birdman to special summon himself, then you overlay him with another Level 3 to XYZ summon Number 17. My question is, does he get banished when Number 17 is summoned since being an Overlay Unit is tecnicly not being on the field so he would be leaving the field, or can he just not be detached since detaching is sending it to the graveyard and he must be banished?