Abstract: Leucism or partial albinism is defined as insufficient production of pigment melanin indicated by normally pigmented eyes, but feather is white to pale brown and pale feather coloration may vary in extent, while some body parts e.g. beak, legs, skin may be normally pigmented. Using citizen science data from Slovak birding site (birding.sk, n = 16 leucists/233 total Blackbird records) and our own observations (n=19), we tested frequency of occurrence of leucism in Slovak Blackbird populations among sexes, habitats, seasons as well as the percentage extent of pale feather coloration. We separately analyzed citizen science and our own data set and found similar patterns. Four categories (0–25, 25–50, 50–75, 75–100%) of the extent of pale feather coloration were used in our analyses. Leucism was more frequent in males than females (73.3 vs 26.7%), all leucistic individual observations were from urban environments, and the observations during nonbreeding season were more frequent (88.6 vs 11.4%) than during breeding season. The most frequent extent of pale feather coloration was in the first category (0–25%), both in the internet records (75%) and in the whole sample (65.7%). Four leucistic males were observed for longer than one year in the same site, while two of them for more than three years. Our results are in line with published study and meta-analysis on a global scale. This is the first study and review on a leucism in a bird species from Slovakia.

Key words: clutch size, brood size, fledgling number, breeding success, nest-box size, altitude