Northern Hemisphere species generally respond to global warming by withdrawing from the southern margins of their distributions and shifting their breeding sites northwards. However, the situation regarding the breeding populations of Goosanders Mergus merganser merganser in Europe appears to be paradoxical: in many places, an increase in the breeding population and range extension have been documented. We studied the breeding biology of this species during the breeding seasons of 1987–1996. We show that the breeding success of a Goosander population breeding in Northern Poland was higher following winters when the ice cover disappeared earlier. There was a significant positive relationship between the disappearance of ice cover and the clutch initiation date, and the breeding success of early broods was higher. The overall mean clutch initiation date was 13 April ± 15 days (SD). The yearly means of clutch initiation dates varied from the beginning of April to the beginning of May. It seems likely that, among other factors, the Goosander's adaptability in terms of the timing of breeding may be partially responsible for the expansion of the species into new areas in the last 30 years.