In the Dark-Eyed Junco, the Offspring of Females and Their

In the dark-eyed junco, the offspring of females and their extra-pair mates are more likely to survive to reproduce, and have higher reproductive success, than the offspring of females and their pair-bonded mate. While this could be explained by the good genes hypothesis, it could also be the result of maternal effects (females invest more in eggs sired by extra-pair mates). Describe how you could test if these offspring differences are due to good genes or maternal effects.