A fox will take chickens at any time of the day they feel safe. They will also take several birds in one day and not leave anything behind because they dart in, grab a bird, and then dart out with the bird still alive, they then kill it and cache it out in the forest and come back for the next bird. If your birds disappeared within a few days and there was little to no feathers then it was definitely a fox. Well, unless a human took them.