I don’t know what your screen cover looks like or how secure that brooder is. A snake could easily eat chicks that age. It doesn’t take much of a hole for the snake to get in but it takes a larger hole for it to get out with chicks inside it. It is possible it was a constrictor type snake that squeezed that chick to death but for some reason decided to not eat it. If there are no marks, injuries, or blood on that dead chick, a snake is a real possibility. It will probably stay away for a few days while it digests the ones it ate then will probably return. The size of the snake will determine how many chicks it can eat at one time. For it to eat three one-week-old chicks I’d expect a snake longer than 5 feet.
Did you closely examine that dead chick for marks or injuries? The only things in the United States I’m aware of that normally kill without leaving injuries or don’t leave evidence other than snakes are canines like dogs, foxes, and coyotes or humans. That padlock probably means it wasn’t one of them. Besides I would not expect them to leave any behind. I don’t know where you live so I don’t know if there are any other likely suspects.
One week old chicks can jump and maneuver surprisingly well. Again, I don’t know what that brooder looks like but obviously there is a hole somewhere if a human wasn’t involved. If a snake can get through that hole after eating chicks then it is possible a chick could get through that hole on its own. It depends on how high that hole is. I don’t have a clue what your area looks like. Normally I’d expect to find the chicks if they managed to get out, they should not go far and should be peeping, but maybe something else got them. That still doesn’t explain the dead chick. From what little I do know if sounds like a snake is the likely culprit but I am not sure.
