It is a modifying phrase considering of a preposition and it's object a noun out a pronoun. Hope this helps.
The answers are:
1) C - sweeter
2) C - the youngest
3) A - louder
4) A - a less expensive
5) D - the worst
6) A - earlier
7) B - further
8) A - more red
9) D - more slowly
10) B - less likely
11) D - quicker; more probable
12) C - later; louder
He is describing late spring/early summer, more specifically a June night.
In the text, it says "On such a night of June". He describes the "little birds", the "far-off wind", and the gushing water. These are all images of early Summer. It seems as though the speaker is outside enjoyin gthe night, looking at the stars, and listening to the birds. Often the sound of birds and nature is attributed to late spring when the animals are full of excitement for the bountiful seasons of spring and summer. Technically the month of June is both in spring and summer as the middle of June is the time of the summer solistice, the longest day of the year, which denotes the official start of summer.