You may need plenty of vertexes in the corners of the mouth, to model the lip corners accurately if you want to make a morph to make the corners of the mouth stretch around something big which it is swallowing, e.g. when a duck is swallowing a frog.

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Remember that the jaw hinge point is not at the exact end of the visible mouth opening. In front of the bony hinge point is part of the jaw articulation and also tendons of jaw-closing muscles, and skin, and perhaps a thickness if fur or feathers. I found that fault in the mouth of swan model that I found, and I had to remedy the matter in my own copy of it/