I am so, SO sorry you have to deal with this!
From what I've seen, foxes will sometimes kill as many birds as they can, take what they need and come back for the rest later. They are generally much quieter than dogs, while attacking. Even so, the widespread area and the chunks missing from some of the birds still point to dogs. It sounds like they stuck around long enough to make a game of it, while foxes are generally in-and-out.
Since this isn't the first attack, whatever came for lunch will likely be back for another meal. The best way to tell (other than sitting out there all day and night with a shotgun) is to set up a camera. If you have a baby monitor in the house, set it up in your hen house, tonight. If your thieves were foxes, they'll likely come back at night. As soon as you can, you may want to rig up some electric wire along the bottom edge of your fence.
Good Luck - and keep us posted.