天鹅monogamous不一定是whole life的。
From Wiki: http://en.wikipedia.org/wiki/Monogamy#Mating_system
Monogamy is one of several mating systems observed in animals. The amount of
social monogamy in animals varies across taxa, with over 90 percent of
birds engaging in social monogamy but only 7 percent of mammals engaging in
social monogamy. With birds the locomotion method has meant that the sharing
of genetic material with non-local sources is far less difficult, and
reproduction is far more successful w
