Abstract. Urbanization leads to the biotic homogenization. Some species adapt, others do not tolerate the changes caused by urbanization and disappear. One of the more tolerant species seems to be the Goosander Mergus merganser. In 2019–2023 (during 755 days/checks in February–May), we analyzed its breeding behaviour, abundance, location of nests, breeding phenology and breeding success in the Spa Park of Piešťany in western Slovakia (n=20 nests on nine trees). Pair behaviour prior to copulation and incubation and adult food were described. Copulations started on February 11 (n=11 pairs), the mean 1^st egg date was recalculated on March 11 (n=9 nests), young (average  number 10.7/ nest, n=9 nests) left the cavity on average April 24, which is earlier as in the north of the species range. All the nests (n = 20) were located on nine Populus nigra poplars at an average height of 19.5 m, when the mean distance of nest from the nearest operating building was 46 m and its mean distance from the water level 117.5 m. Predation of young by crows and gulls, and tolerance to hetero- and homospecific nesting on the same tree were also described. In the diet of adults, mainly smaller fish (< 10 cm length) were found, e.g. Alburnus alburnus (80%), more rarely bigger fish >20 cm, e.g. Rutilus rutilus (7%), small freshwater mussels <3cm (10%), and the aquatic plants only seldom (3% of observations).

Key words: water birds, urbanization, phenology, breeding biology, food, Slovakia