Final answer:
This is an example of intersexual selection, where females choose mates based on certain characteristics. Female birds in this case choose to mate with males that have bright red colors, which is energetically expensive to maintain.
Step-by-step explanation:
This is an example of intersexual selection. In intersexual selection, individuals of one sex choose mates of the other sex. In this example, female birds choose to mate with males that have bright red colors, which is a secondary sexual characteristic. Because maintaining bright feathers is energetically expensive, only the healthiest males can sustain this trait.