IN ACT I SCENE 6, THE SCENT OF WHICH BIRD SOOTHES BANQUO AND KING DUNCAN WHEN THEY REACH MACBETH'S CASTLE ?

It is summer. Duncan is probably calling attention to the
fragrance of flowers, grass or some other natural fragrance of the summer air. This
illustrates his peaceful disposition and the calm after the
battle.

Banquo mentions a bird, which is a martin, but
could be a swallow or swift. The bird is first mentioned as the “temple-haunting”
martlet. This could refer to a bird on a coat of arms but probably refers to an actual
bird perched on one of the rafters. The martin builds nests in high places such as
cliffs and inside cathedrals or castles.

Banquo
says:

The
temple-haunting martlet, does approve,

By his loved
masionery, that the heaven’s breath,

Smells wooingly here;
no jutty, frieze,

Buttress, nor coign of vantage, but this
bird,

Has made his pendant bed and procreant
cradle:

Where they most breed and haunt, I have
observed,

The air is
delicate.

The temple-haunting
martlet is the martin. The martin approves of the peaceful air, otherwise he would not
have chosen to nest there. The martin often perches and nests in the rafters of
cathedrals. This is the reference to “heaven’s breath.” Since the martin has made a
nest, Banquo notes the air smells “wooingly,” meaning the martin will mate. The
“procreant cradle” refers to the birds nest. Banquo concludes this is a good omen since
these birds tend to breed and haunt (live in) peaceful or religious places like
cathedrals.