Aosagi-bi

Aosagi-bi is a nocturnal phenomenon in which aged grey herons are said to emit a pale blue, phosphorescent glow from their bodies, beaks, and wings as they fly at night. The effect is described as resembling foxfire or will-o'-the-wisp. This being represents a characteristic early modern period fusion of bird and flame imagery in yokai *depiction, with a parallel tradition attaching to the five-rank heron (goi-sagi).

The phenomenon appears in Toriyama Sekien's *Konjaku Gazu Zoku Hyakki* (1779), where a grey heron is illustrated radiating blue light against the night sky. The accompanying text notes: "The aged five-rank heron, flying by night, casts forth light"—attributing the glow to phosphorescence from a long-lived nocturnal bird. The image reflects observation of waterscapes in the early modern period Edo Bay wetlands and tidal regions, particularly in the Sumida River valley (Musashi Province) and the Kasukabe and Ryōgoku areas, where it appears scattered through period essays and haiku.

The tradition is documented in Terajima Ryōan's *Wakan Sansai Zue* (1712), a natural history compendium that records accounts of the grey heron's nocturnal luminescence. The name appears in Toriyama Sekien's work as the earliest known textual reference. In the modern period, the phenomenon was systematized in reference works such as Murakami Kenji's *Nihon Yōkai Daijiten* (Kadokawa, 2005) and entered the International Research Center for Japanese Studies' "Kaii and Yokai Tradition Database."

Aosagi-bi shares genealogical ties with other bird-and-flame yokai such as "five-rank light," owl-fire (fukurou-bi), and the nue, all linked through a visual framework of avian-luminous fusion. Accounts of phosphorescent water-bird phenomena in the lower Tone and Sumida river wetlands of Tokyo, Chiba, and Ibaraki Prefectures persisted into the modern period.