Sexual selection

Females are selecting males for slaughter. Of course, it's not intentionally, it's just that females which are selecting males for slaughter give their genes better chances of survival, by combining them with the genes of the best males around. They can obviously select themselves which male has the best genes, but it's even better if the selection is automatic, like them dying because of being conspicuous and predators catching them. You can well imagine that peacocks are very conspicuous. 

Imagine that there are two groups of birds from one species. One group is monogamous, in the other there are far more females than males, so they are polygamous. The reason why there are more females than males in the second group is that they have selected conspicuous males and they have mostly died because of predators. Which group would be more successful? It really depends. The first group has the advantage of having more offspring, because the male helps in their rearing, let's say that the first group has two times more offspring than the second group. The second group has the advantage that males are highly selected, so only the best genes survive from males, therefore, their offspring have higher chances of survival. You may imagine that there are ten times more males in the first group than in the second. One male can easily copulate with ten females.

If the female offspring from the polygamous birds have two times more chances of survival than the female offspring from the monogamous birds, than we have an equilibrium, the populations remain stable. If they have three times more chances of survival than the monogamous group, over time they will take over and all their species will be polygamous, with conspicuous males. If they don't have at least two times more chances of survival, then the monogamous group will take over in time, their males being inconspicuous. 

In nature we can observe both strategies. The pigeon, for example, is monogamous. Some pigeons may well be colourful, thus, conspicuous, but their ancestor, the rock dove, is pretty inconspicuous. On the other side, we have the famous peafowl, which are polygamous, because the peahens are selecting peacocks for slaughter, so there will obviously be more females than males, therefore, polygamy is necessary. Another lovely example is the mandarin duck, compare the male with the female.

You can well imagine that a conspicuous bird is attacked all the time and if it manages to survive, that's a far more impressive achievement than the survival of an inconspicuous bird. Basically, it needs to be healthy, fast, strong, etc. These are great qualities for females too, this is why they are selecting those males for slaughter, because only the best survive and give them the best genes, thus, the next generation of females has bigger chances of survival than the generation of females from those who select inconspicuous males.