Aosagibi or Aosaginohi (Blue heron fire) (青鷺火)
Aosagibi is defined as a mysterious phenomenon in Japan in which the body of a heron emits a pale blue light mainly at night. It is also called Goi no hi (blue heron fire) or Goi no hikari (blue heron fire).
Although it reads "Aosagi", it is not aosagi (grey heron) but is said to indicate goisagi (young night heron).
It was also featured in a well-known collection of specters illustrations in the Edo period Konjaku Gazu Zoku Hyakki (Continued Illustrations of the Many Demons Past and Present) by Sekien TORIYAMA and in Ehon Hyaku Monogatari (Picture Book of a Hundred Stories). It is thus evident that it was quite common as a Kaidan (a ghost story) in the Edo period. Also, in "Bakemono Haruasobi" (monster's spring play) written by 桜川慈悲功, a writer of gesaku (literary work of a playful, mocking, joking, silly or frivolous nature) in the later Edo period, there is a story in which a luminescent grey heron was seen in Yamato Province (Present Nara Prefecture). According to the story, blue fire was seen almost every night at a large willow tree called "bakeyanagi" (willow monster) and was feared by the people. And on one rainy night, when a man came near to it saying "The fire may not inflame on a rainy night", the whole tree began to shine in blue. The story tells that the man fainted due to fear. This phenomenon of a mysterious light is said to be the act by a grey heron. According to the legend in Niibo-mura, Sado ga-shima Island, Niigata Prefecture (Present Sado City), a dragon lantern (mysterious fire which is said to be lit by a dragon) came flying almost every night at a plum tree in the Konpon-ji Temple. When someone shot it with a bow and an arrow, it was reportedly found to be a heron (Ardeidae) in itself.
There is transmission of a legend that the features of those birds in the mountains including young night herons, ducks, and pheasants, etc., emit a light when they fly at night. There are not a few cases of witness accounts. Kimimori SARASHINA, a researcher of a native town wrote in his book entitled "Hikaru tori, Hitodama, Hibashira" (shining bird, disembodied soul, pillar of fire) about many firsthand accounts of those grey herons and young night herons shining in pale blue like aosagibi (blue heron fire), including a story in which a young night heron was seen to shine in pale blue in Ibaraki Prefecture in around 1928. A heron is said to become a ball of flame. There are also legends that say that it flies with a flamed branch in its mouth or that it exhales fire. There is a witness account in which a young night heron was reported to be seen to exhale fire on the surface of the Tama-gawa River. There is also a description in an encyclopedia compiled in the Edo period "Wakan-sansai-zue" that reads that the appearance of a young night heron flying in the air is just like a fire and is especially seen brightly on a moonlight night. The description reads that people may possibly identify it as a "yokai" (specter) by mistake.
On the other hand, there is also a legend that young night herons disguise when they become old, similar to red foxes, raccoon dogs, and bake-neko (a monster cat). There is a hypothesis that this was because young night herons are active at night and because the appearance of its flying in the night sky and singing aloud may have given a weird impression to people. An old goisagi comes to have uroko (scale pattern) on its chest and it comes to look chalky and yellow. In around autumn, it is said to fly in the cloudy sky emitting a pale blue light.
Scientifically, a major hypothesis suggests that luminescent bacteria existing in aquatic environment attach to the body of a bird and are seen to shine in moonlight at night. There is also a hypothesis that white feathers growing on the chest of young night herons look shining when seen by the eyes at night.