The ancestor of these birds was none other than a plain crow like bird. Thanks to evolution and isolation we have the beautifully birds we have now. This evaluation may be caused by natural and sexual selection. Also it might included speciation.
Natural selection is when a mutation causes survival of fittest and the mutation is passes to the next generation.Every generation with the trait will have or inherited it until the whole bird population has it.
Sexual selection is when a mutation causes traits to increase the like hood of mating.
For example, sexual selection may change the same species by how food is spread out. If It's clustered the males would be burly. To defend their territory of food and females. If more dispersed the males will devolved fancy feathers or the trait fancy, since the dispersed food will be impossible to defend.
Speciation will begins when groups become separate in location, or become different enough in form and behavior that one group won't mate from another group.