I am also obligated to post a disclaimer. We don't want to hear about you getting thrown in lockup on suspicion of public intoxication because you were harvesting Pigeon droppings from the local park! Now, don't go do something stupid just because you happened to read about making gunpowder using Pigeon droppings online. You can make your own charcoal, which is the final ingredient to making homemade gunpowder, should you ever find yourself in a situation where you absolutely need homemade gunpowder!

Wikipedia also has an article on Sulphur and lists natural sources. If you are determined enough, you can find natural deposits of sulphur, or you can drop in on your local Mad Scientist's Supply shop and pick some up there. If you are really that curious, this entry in Wikipedia explains how to extract Potassium Nitrate from Guano. The process requires a lot of urine also, and probably a few other elements that many people would rather not handle directly. Through a rather tedious process of composting and leaching, the Potassium Nitrate is extracted from the manure. Many kinds of manure are high in nitrates, and Pigeon manure is one of them. You can extract Saltpetre, or Potassium Nitrate. No, you can't make gunpowder out of Pigeon droppings. Dovecotes were even guarded at one time to prevent thieves from stealing the valuable commodity, and wars were fought over islands that were covered feet deep in seabird guano. But it was used at one time to provide one of the ingredients of gunpowder. You can't exactly take out your favorite black powder firearm and load it up with droppings scraped from old George in the park and expect it to fire.

Making saltpeter from chicken manure