First Documented Case of Complete Leucism in the Long-Eared Owl (Asio otus) in Bulgaria.

Leucism is a hereditary anomaly affecting the coloration of avian species. It is defined by partial or complete loss of plumage coloration. Unlike albinism, which results in ocular depigmentation, leucistic birds have normal iris color. Pigment abnormalities within wild bird populations are rare. In the order Strigiformes, such cases have mainly been recorded as individual reports. This research introduces the first documented case of complete leucism in the Long-eared owl (Asio otus) in Bulgaria. A retrospective analysis of clinical records from the Wildlife Rescue Center ("Green Balkans", Stara Zagora) between 1995 and 2026 revealed that, among 1098 admitted individuals, only one exhibited this phenotype. In February 2016, a leucistic individual with multiple fractures was admitted into the center. The bird displayed the hallmark phenotypic features of complete leucism. Following treatment and rehabilitation, the owl regained full flight capacity and was successfully released into the wild in June 2016. This case confirms the extreme rarity of leucism in the Long-eared owl and highlights the essential function of long-term rescue center databases in documenting rare morphological anomalies and advancing conservation research.