Early in the mating season, as a female bird casts her eyes over the group of males in a bush or strung out along a fence, she will subconsciously pay attention to small variations in the plumage of the different males. Is one male slightly brighter than the others, or does one perhaps have a colored patch slightly larger in size than his fellows? This subtle variation between the males will direct her in choosing a mate for the coming breeding season, or possibly even for the rest of her life. That a female bases such an important decision purely upon the appearance of a potential partner may at first seem quite arbitrary. In fact, variation in the expression of color in birds can signal a wide range of different benefits to a discerning receiver, many of which will have significant effects on the immediate reproductive success and subsequent life-history of that individual.