To me, the fun factor makes as much sense as any of these labored speculations, but there aren't many formulae for fun in evolutionary algebra. As might be expected, there is tremendous variety in the reproductive behavior of birds, and the past decade has seen an intense renewal of interest in the nature of their pair bonds. Some of these long-term field studies, many of them correlated with DNA fingerprinting, are reported in Black's book. Geese and swans maintain the closest pair bonds, with almost no extra-pair conceptions. Barnacle geese fly from the high Arctic in summer to winter on coastal marshes in Scotland and England.
Pairs maintain contact throughout the day, every day, for life.
Iceland's whooper swans are known to mate exclusively within the pair. In contrast Australia's splendid fairy-wrens father only 30 percent of the offspring with their social partner. Even among the high-fidelity mute swans Clive Minton described one dramatic defection among the 100 pairs he watched in Staffordshire, England. After a prolonged battle, a usurper drove the territory-holding male away. Just maximizing her fitness.