Abstract: The breeding behaviour of leucistic birds is poorly known. The Blackbird is the species in which leucism occurs most frequently in Europe. In Slovakia, a total of 35 records of leucistic individuals with different range of white coloration have been published in Slovakia, while complete leucists are rare. In the urban environment of Žilina (NW Slovakia), we studied the breeding behaviour and breeding success of a completely leucistic female, which is the first known case of such a female breeding in this species. From April 20, to September 7, 2024, her behaviour, nesting attempts, nesting success and causes of nesting failure were monitored on the university campus of the University of Žilina with a park-like habitat (for a total of 148 hours during 36 days). Observations were made between 5 and 15 hours CET and documented by photo. The individual moved during the research in a polygon of 17 ha (530 × 320 m). Altogether six copulations (4–12 m from the nest) and five nesting attempts were registered (findings of individual nests on 20. 4., 14. 5., 20. 5., 27. 5., 1. 6.), but none of the nesting attempts were successful. The most likely causes of nest failure were nest predators (2), weather (2) and park management (1). Direct predation was observed in one case by a magpie during egg incubation. Individual nests were located 330, 360, 390 and 410 m apart (distances from the first detected nest), on trees and shrubs at a height of 2–3.5 m, at a distance of 4–7 m from the edge of the stand, 2–25 m from the nearest building, 5–15 m from the nearest sidewalk or road.

Key words: aberrant coloration, passerines, breeding success, urban ecology