Answer:
In the given case, there would have been a decrease in the variation of the shades of red as no form of the trait will be advantageous. The red color of the male cardinal exhibit in order to fascinate the female cardinal.
In case if the female cardinals opt for another means for selecting their mates, then the male cardinals shall possess no application of the red shade. Thus, the variations in the red shade shall diminish because of disuse. Therefore, to overcome the situation, the male cardinals need to adapt to the new mechanism of selection.