A detailed evaluation of specimens of an early bird fossil (Ichthyornis dispar) from America (some from previously unappreciated museum collections but one actually recent) has revealed that they had dinosaur-like jaws with sharp teeth and strong, unbird-like jaw muscles (https://www.theguardian.com/science/2018/may/02/fossil-sheds-light-on-evolutionary-journey-from-dinosaur-to-bird). These early birds (thought to be rather gull-like in size) just had a small toothless beaks (perhaps used for preening and other functions) in front of their actual jaws. It appears that the jaws, along with the teeth, were lost in evolution as the beak extended to become the sole bird feeding organ.
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