K(potassium), P(phosphorus), and N(nitrogen) are found in guano, but that’s any animal(including human). Bird poop and bat poop are just the ones that quickly build up. But the core ingredient of the poop isn’t saltpeter(KNO3), at least not until it starts to crystalize and develop, since coming from the body it isn’t KNO3. And, bird poop isn’t really, the core ingredient of KNO3 because a chemical like it can be made in many ways, but one of the most common historical processes only needed guano, and water(but water wasn’t an ingredient)