Parental care is regarded as one of the most influential forces acting on the evolution of mating systems. Polynesian Megapodes (Megapodius pritchardii) are burrow nesters and rely on geothermal heat for incubation. Because they provide no parental care, either before or after hatching, they can provide insights into selective forces that have shaped mating systems in the absence of parental care. Our study of their mating system—the first such study of any burrow-nesting megapode—suggests social monogamy. The partners of 10 marked pairs stayed together in the same territory for ≤12 months and were seen with their partner significantly more often than on their own. In 64% of all observations, pair partners foraged <5 m apart, and females fed on food items uncovered by the male. Duets also indicated long-lasting pair bonds. We propose that monogamy in this species is related to the fact that females lay only one large egg at a time (which weighs, on average, 24% of her body weight), in intervals of several days or weeks. Producing eggs year-round, females require a high intake of protein-rich food. They seem to benefit, within the pair bond, from the male-defended feeding territory and from the invertebrates the male uncovers while feeding close by. While being guarded by the male, females can spend more time searching for food and are protected from forced copulations by other males. The monogamous pair bond seems to benefit the male by enhancing his chances of fertilizing his partner's eggs; with an unpredictable laying interval and year-round egg production, males cannot predict when their partner will be fertile and thus benefit from staying with her year-round.