Donk:
I hope for the females' sake these threesomes are as harmonious as they appear to be, as male mallards are notorious for mating with
any ducks - not necessarily willing, female, of the same species, or even alive.
They also do not form pair bonds. If these threesomes are indeed permanent or semi-permanent groups (rather than two repeat rapists and their victim), further study of this community is certainly be called for.
The sordid truth about mallards... 
Galapagos:
Birds rather differ in their minimal investment from mammals in that the absolute dependence of the offspring on the mother (rather than on any parent) ends when the egg is laid, whereas young mammals depend specifically on their female parent for the duration of gestation and for milk after birth. This makes polyandry more likely to develop in birds than in mammals, although it doesn't actually explain how it came to be in the first place. One possibility I can think of could be a shortage of females in the past leading to selection for a breeding strategy that allowed each female to lay many more eggs than she could brood herself or with the help of only one male, but it is difficult to visualise conditions that would lead selectively to a shortage of females.
