I had an opportunity to question a 343 employee about this last night. (Can’t say who though)
Anyway, I ask if the change to the Covies looks were for aesthetic reasons, or polygon count, or something entirely different.
As suspected, the look of the traditional Covenant forces was changed to make them look more menacing. Frank O’Conner has eluded to this as well.
There apparently was a discussion about the Jackal in particular with their more bird-like appearance and not being as threatening. The decision was made to make them appear a little bit more dinosaur like, giving them a stunted beak and moving their eyes forward.
That definitely changed the look of them. For the better? Hmm you decide:
I happen to like it. Other than the head, much of the rest of the Jackal remains intact from previous iterations. The new version in Halo 4 definitely looks more bad-ass and no where near bird-like.
I was told there really won’t be any canonical explanation for this. It’s not like the evolved. It’s just an aesthetic change.