Final answer:
Intersexual selection occurs when females choose mates based on traits like bright plumage, which may not enhance survival but increase reproductive success.
Step-by-step explanation:
The phenomenon described in the student's question, where the plumage of male birds becomes brighter over time due to female choice, is known as intersexual selection. This type of selection occurs when individuals of one sex select mating partners based on certain desirable traits in the other sex, which often include visual cues like bright plumage, aural cues like mating calls, or the performance of elaborate dances. The preference for these traits, though sometimes detrimental to survival, can lead to increased reproductive success for the males possessing them, perpetuating the traits in subsequent generations.