How can extra-pair mating benefit female blue tits and their offspring?

In the context of reproductive behaviour, female blue tits (Cyanistes caeruleus) that engage in extra-pair mating—accepting copulations from males other than their social mate—often produce faster-growing offspring. This suggests that such behavior provides significant genetic benefits to the female's brood.1

While many birds are socially monogamous, DNA fingerprinting has revealed that females frequently seek extra-pair copulations. These matings may allow females to "trade up" in genetic quality, potentially securing "good genes" that enhance the performance, growth, and survival of their offspring. Additionally, multiple mating can serve as an insurance strategy against the possibility that the sperm from a primary social mate is inviable.1