Communication Needs

What are the methods of long distance communication with low technology on an island?

Considering it is 1821. You are strained on an island. Which means resources and technology are low. You are around 10km apart from each partner.

Public Comments

1. Flares, smoke signals
2. mooning each other when my wife moons me its like seeing an actually moon it is so bright and big
3. Drums made from hollow trees. Runners in relay could carry messages. Smoke signals. Horns made from animal tusks. Flags, semaphore, done from cliffs. Mirror signals. Carrier pigeons. I don't know how many partners are on the island. Relays need people.