Die, Devil Bird! will be your third Boss Fight.
Infected Bat, Giant Bat.
You will meet the Infected Bat in the Church after you crossed the Bridge from the Observatory.
On easy it only took about 4 shots from the Shotgun to kill him.
After the first shot he starts spawning little Bats. Kill 16 and unlock...
Also make sure you let him pick you up to unlock...
I will update the number of shots it takes to kill the Infected Bat on higher Difficulty later on.
I take no Credit for the Video.
Hope it helps