How does Patrick O'Brian use imagery in The Commodore?

Imagery:

The moon had set: there was barely a star to be seen. The still air was delightfully fresh with falling dew, and a late nightingale, in an indifferent voice, was uttering a routine jug-jug far down in Jack's plantations; closer at hand and more agreeable by far, nightjars churred in the orchard, two of them, or perhaps three, the sound rising and falling, intertwining so that the source could not be made out for sure.