Spreading wild bird poop on the ground to use as fertilizer is one thing, but gathering it to put on your face is another. You might imagine people tracking nightingales and running around to scrape the poop off various surfaces, but there's a very specific process that goes into making bird poop into an ingredient. There are nightingale farms in Japan that exist solely for the purpose of harvesting the poop. Wild nightingales eat things like insects and berries, but at the farms, the birds are caged and fed a diet of organic seeds.
After their poop is collected from the birdcages, it is sanitized, often by using an ultraviolet light. After the poop has been dried out (usually with a dehydrator), it's ground into a very fine white powder. This powder is sold to companies, who in turn sell it to spa owners or shop owners who cater to geisha. Sometimes it's mixed with another product, such as rice bran, for exfoliation.
To get the facial, the powder is mixed with water to form a paste, and then massaged into the skin for a few minutes before rinsing it off. Users describe it as having the same effect as a light chemical peel, but without the redness and burning -- their skin feels clean, soft, moist and not at all irritated.
Wild bird poop can contain bacteria, fungal spores and other unhealthy substances, but uguisu no fun is safe and clean due to the birds' special diet, controlled habitat and the purification process. So even if it's not a miracle skin product, it's probably not going to hurt you. Some users caution, however, that it's not for very sensitive or allergy-prone skin, and that you should be careful to avoid getting it into your eyes, nose or mouth.
That's all well and good, but why exactly did bird poop facials come into being in the first place? Next, we'll look at the origins of uguisu no fun.