Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior. In other words, before an animal engages with a potential mate, they first evaluate various aspects of that mate which are indicative of quality—such as the resources or phenotypes they have—and evaluate whether or not those particular trait(s) are somehow beneficial to them. The evaluation will then incur a response of some sort.
Mate choice is highly visible in lek mating. Here, black grouse males gather in a quagmire and the females then arrive and observe the male before choosing one.
Ronald Fisher in 1913
The peacock tail in flight, the classic example of a Fisherian runaway
A male satin bowerbird guards its bower from rival males in the hopes of attracting females with its decorations.
Peafowl is a common name for two bird species of the genus Pavo and one species of the closely related genus Afropavo within the tribe Pavonini of the family Phasianidae. Male peafowl are referred to as peacocks, and female peafowl are referred to as peahens.
Peafowl
A peacock spreading his tail, displaying his plumage
Peahen
Peafowl eggs