Studio has its own project
Given its prior work on the franchise, it wasn't unreasonable to expect Sledgehammer Games to be involved with Call of Duty: Ghosts. That does not appear to be the case, however. "Congrats to Infinity Ward on [Ghosts]! Our team is heads down on our next project, but best wishes to the hardworking team at IW!" reads a tweet.
Before stepping in to co-develop Call of Duty: Modern Warfare 3 alongside Infinity Ward, the studio had a third-person Call of Duty title in production. I'm hoping Sledgehammer can return to the concept if for no other reason than to help break the monotony. More recently, we reported that it was hiring for a senior environment artist to work on the franchise.
So, something CoD-related is going on at Sledgehammer, and it's not (or at least not exclusively) Ghosts. That makes how many studios actively developing these titles? My head is spinning.