Zac Gallen is used to doing a pretty good Randy Johnson impression on the mound ... but on Wednesday, he imitated The Big Unit in a way no pitcher ever wants to -- by killing a bird with a pitch.
The Diamondbacks ace threw the unfortunate toss as he and other Arizona players were warming up before their afternoon game against the A's in Oakland.
You can see in video captured by a camera stationed in the Coliseum's outfield ... as Gallen rocketed what appeared to be a curveball to his catcher -- he inadvertently struck a bird who was flying nearby.