noun / verb
sip
SIP/sɪp/
A small mouthful of a drink, or to drink in small mouthfuls.
She takes a small sip from the teacup.

Etymology
Middle English sippen already meant drinking little by little, so the small-mouthful sense is longstanding. Its earlier origin is uncertain; one possibility is a modification of sup intended to suggest a slighter action.
In context
they can see the great white flowers bright in the light of the full moon and they go to them to sip the nectar and in doing so pollinate the cactus