Distribution and expansion of the breeding population of Goosander Mergus merganser in Switzerland

A relatively isolated breeding population of Goosander occupies the Alpine region in Europe. Historically, Lake Geneva has played an important role in the spread of the Goosander population, especially after hunting of the species was prohibited in Switzerland and in France. In Switzerland, there has been a marked range expansion, which is well documented by atlas projects and special surveys. An increase in population size of 21% was noted between 1998 and 2013–2016, showing the colonisation ability of Goosander and its higher tolerance of humans than in the past. However, range changes varied between regions, with an important decline in the western bastion and an increase in the northeastern part and south of the Alps. The decrease of some fish populations, and the removal of old nesting trees and increased predation pressure from Yellow-legged Gulls Larus michahellis may explain regional declines.