The differences of size, color, voice and so on between the two sexes in different animal species is known scientifically as sexual dimorphism.
The phenomenon is well expressed in birds, were normally the males are bigger, more vividly colored and possess a more melodious voice compared to females.
Causes that induced this have acted along the evolution of the species.
In monogamous species (that form couples) (91 % of the bird species) the bright color of the male appeared as a necessity to make themselves noticed on the territory they occupied for nesting, for the optical marking of that territory.
Placed in visible places, these males warn other males from their own species that the territory is occupied, to keep them away and for attracting easier the females in their territory.
The monogamous males also usually possess a strong or melodious voice for the same goal: sonorous marking.
In the case of the polygamous species (where a male mates with several females) (8% of the bird species) the vivid color of the males is even more stressed, sometimes only during the mating period.
Here too, the bright plumage is an optic signal for intimidation and keeping away other competing males because the male with the brightest plumage will dominate the "battle field" and to mate with as many as possible females gathered for mating.
In the case of the polygamous species, the color dimorphism is often accompanied by size dimorphism, males being bigger.
The result is that the most robust individuals will eliminate from reproduction the lousier males.
This is clearly beneficial for the species, and the natural selection maintained it.
Examples of polygamous species are pheasants (above the Argus pheasant, peacocks, widow birds, paradise birds and bowerbirds.
But the dimorphism can be sometimes an advantage for females, when they can be bigger and brighter.
This is the case of polyandrous species (< 1 % of the bird species), when the female mates with more males.
As for the polygamous species, the male is the active sex, in this case is the opposite: the females are the active sex.
Females occupy the battle field and chase away their weaker female rivals.
Subsequently, only the bigger and brighter females will be fecundated by the males gathered around them and their descendants are more vigorous than the ones mothered by their rivals.
Examples of polyandrous bird species are buttonquails (center, a pair of buttonquails, female is larger and darker), jacanas and some shore birds.
But there are cases of monogamous species where a sharp size dimorphism does exist in the favor of the female.
This is found only in diurnal birds of prey, like falcons, hawks, buzzards, kites, harriers, eagles, and others (photo bellow, a pair of African fish eagle).
But why did this strange situation at first sight appear?
The chicks of the birds of prey have a nidicole development: the offspring stay in the nest till their complete development, being fed by the parents with live prey.
When they are small, the prey species are also smaller and these preys are more suited for the hunting ability of the smaller males.
At the same time, the bigger female is more efficient for protecting the more vulnerable young.
As the offspring grow, the bigger female, able to bring down bigger prey items, is more efficient in delivering food for them.
This way, during the evolution of these species, a "work partition" produced sexual dimorphism through natural selection.