Yes they can.
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First, we need to know what [Gunpowder][1] is. Luckily we have Wikipedia, they will need an inventor. Will make a good story on how to get to gunpowder, methinks.

Your island(s group) are very lucky. A few of them have huge bird colonies. Those big bird colonies create a lot of [guano][2]. So you can feed a lot of people from the small area you have. Good, you will need your people to be numerous. But you can create part of [gunpowder][3] from [guano][4].

Next step is sulfur. Again, you have hit the jackpot! A volcano with a [Sulphur Mine][5]. You might want to create a lower caste to get this stuff. It really smells. You know, like hell or another bad place.

The last step is charcoal. Now that is easy, we get that from burned wood. Now wood will be a bit of a resource bottleneck on an island. But for the greater boom, it will need to be done.

**Oke, now we have Gunpowder, how do I use it?**

In the west, it is best known for its use in cannon and rifles. Those things are not available to you, for lack of metal (working).

So I suggest, go with mines and grenades. Both will need [fuses][6]. You have the material to make those, so no biggy. 

**Mines** are best used for guerrilla activities. Blow up their boats when they are close to the shore. Explosions work better under water. You do need to do some waterproofing. I think you can use animal fats for that (thank you birds).

**Grenades** will be a tennis to basketball size affair. An explosion is nice, but it is the shrapnel that will kill. So use a hollowed out coconut for the shell. Make it as thin as possible. Then line it with stones. Sharp ones are preferred. In the middle, you place the gunpowder. Attach fuse, light, and throw at an enemy. Make those foreigners sorry they ever came this way!


  [1]: https://en.wikipedia.org/wiki/Gunpowder
  [2]: https://en.wikipedia.org/wiki/Guano
  [3]: https://en.wikipedia.org/wiki/Potassium_nitrate
  [4]: http://pigeonsformeat.com/index.php/utility-pigeons/10-gunpowder
  [5]: http://www.bbc.com/news/world-asia-pacific-12301421
  [6]: https://en.wikipedia.org/wiki/Fuse_(explosives)