Poo to you.

Although pigeon poo is seen as a major problem for property owners in the 21st Century, it was considered to be an invaluable resource in the 16th, 17th and 18th century in Europe.  Pigeon poop was a highly prized fertiliser and considered to be far more potent than farmyard manure.  So prized in fact that armed guards were stationed at the entrances to dovecotes (pigeon houses) to stop thieves stealing it!  Not only this, but in England in the 16th century pigeon poop was the only known source of saltpetre, an essential ingredient of gunpowder and was considered a highly valued commodity as a result.  In Iran, where eating pigeon flesh was forbidden, dovecotes were set up and used simply as a source of fertilizer for melon crops and in France and Italy it was used to fertilize vineyards and hemp crops