Racoon, Fox, coyote.
Being the door was small leaves me to think less of a coyote, but not totally rule it out.
Fox will take prey and remove it and go back get more, also eat bird whole.
Racoon can run in family units or a pack, but also be solitary. Also eat a bird whole.
Chicken size door...
Murder mysteries need clues.
Need details about the coop. How did the intruder enter if in a coop? Was there any prints?
Was the coop enclosed? the animal broke in somehow, was it fenced? animal climbed or jumped over?
I am guessing where you live is not extreme cold in winter to where animals...