Why is social monogamy common in birds but less common in mammals?

The prevalence of social monogamy in birds compared to its rarity in mammals is primarily driven by the differing requirements for parental care to ensure offspring survival.12

In most mammalian species, the female provides the essential care through gestation and lactation. Because the mother is physiologically constrained to provide this care, the male’s contribution is often unnecessary for the survival of the young, allowing him to desert and seek additional mating opportunities. Consequently, polygyny is the dominant mating system in mammals.12

In contrast, many bird species require biparental care to successfully raise their young, particularly when environmental resources are limited. In these cases, the survival of the offspring is significantly higher when both parents contribute to tasks such as feeding and nest defense. Natural selection favors social monogamy in birds when the maximum survival of the brood necessitates the combined efforts of both the male and the female. While extra-pair fertilizations are common in socially monogamous birds, the benefits of shared parental investment generally outweigh the advantages of male desertion.12