Perhaps the familiar swallow has never been treated with more discrimination; and at this point of the tale of horror it has the effect of a ray of sunshine in a sky dark with storm clouds.
In Act II. Macbeth describes his own horror and Nature's:
Now o'er the one half world
Nature seems dead.... Thou sure and firm-set earth,
Hear not my steps, which way they walk, for fear
Thy very stones prate of my whereabouts.
Lady Macbeth says:
It was the owl that shriek'd, the fatal bellman
Which gives the stern'st good-night.