Mating among birds is usually a matter of mutual agreement: both male and female are being selective in their choice of mates (although their criteria is likely to differ). Selectivity in mate choice is central to the Darwinian process of sexual selection, determining which individuals will mate and produce offspring. The chosen individuals are more successful in mating and reproduction. Thus, their genes are passed on to the next generation while the genes of un-chosen individuals are not. As the process continues, the genes that determine both the winning traits and the choosiness become more frequent in the population.<\/p>\n
The outcomes of all that choosing vary enormously, depending on the ecology of the species, the previous evolutionary history and the occurrence of genetic variation upon which selection can happen. If there is no plumage variation, there is nothing to choose. Avian feathers serve various functions, one of which is visual display during courtship. Genetic variation in plumage among potential mates provides birds with choices of color and pattern as they decide with whom to mate. Those choices, together with variations in other traits, shape the appearance and behavior of the lineage.<\/p>\n
[This is how the animal kingdom reproduces]<\/a><\/ins><\/p>\n Imagine a species (Species A) that makes open-cup nests (as do all related species … that’s an evolutionary history factor) in shrubs (instead of under logs or roots — that’s an ecology factor). Suppose that in this species it is (for whatever historical reason) the female that incubates the eggs and cares for the nestlings. In an open-cup nests (compared to a cavity nest, for instance), the incubating female would be exposed to searching predators and her frequent feeding visits to a particular location would also be noted by lurking predators. So inconspicuous, perhaps camouflaged, plumage would lead to better survival of the female, the eggs and chicks. And a male that preferred an inconspicuous female would have better reproductive success than one that somehow preferred a female that flashes a noticeable red crest or a long, bright blue tail.<\/p>\n If there is some variation among the males in the colorfulness of their plumage, those drab females might prefer males that are a bit colorful, perhaps with a yellow head or vivid magenta wings, rather than males that are more like the females. If those plumage patterns are heritable, the sons and daughters of such adults would bear the same traits, and eventually the whole population would have drab females and colorful males.<\/p>\n However, if (in Species B) the males also do some of the incubation and parental care, they and their eggs and nestlings might suffer more predation. If they were colorful, then they and any female that somehow preferred a gaudy male would probably have lowered reproductive success. And so their genes would become less frequent in the population, and males and females would look similar in color.<\/p>\n