A female is likely to fall for a deep-blue male, even if she already has a mate, and especially if this mate is pale blue. Neither does the deep-blue male hesitate even if he already has another female. One-third of all broods include at least one chick whose father is not part of the "marriage".
Additional offspring, of course. In this way, the male spreads his genes more effectively to the next generation.
The female is likely to have some sons with deep blue caps. They are attractive to females of the next generation, which means that her genes will also be spread more effectively.
The loser in this triangle drama is the female's less attractive partner, the male with the pale blue cap. His mate cheats on him, and he is unaware that some of the young he is feeding are not his own.