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.
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.