Alertly

"Behind one of the old oaks which we have described in the preceding chapter, shrouding himself from observation like a hunter watching for his game, or an Indian for his enemy, but with different, very different purpose, Tyrrel lay on his breast near the Buck-stane, his eye on the horse-road which winded down the valley, and his ear alertly awake to every sound which mingled with the passing breeze, or with the ripple of the brook."