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.