A pair of swans, slender necks entwined, might be one of the most romantic images humans have appropriated from the animal kingdom. Indeed, the vast majority of bird species -- about 90%, a rate far outstripping that of mammals -- form monogamous pairs to mate and raise young.
But given the right opportunity, according to a new report by a group of U.K. researchers, once-monogamous birds will turn to polygamy and promiscuity. The scientists examined 197 different bird species, including swans, to determine why birds made the switch.
They found that when a population of birds skews heavily toward males, female birds are more likely to cheat on their mates. And when females are more abundant, the "divorce rate" -- the number of male birds who leave their mates to court better matches -- increases. "Basically, the rarer sex has more opportunity to ‘play the field' and either cheat on the partner or leave in favor of a new mate," says András Liker, an ornithologist at the University of Sheffield.