Why do females increase parental effort when caring for the offspring of attractive males? First, attractive males may be poor fathers so that their females are compelled to increase their own contribution in order to fledge some young (the partner-compensation hypothesis). Second, females mated to attractive males may be willing to increase their parental effort to reap high indirect benefits for their offspring, and in turn males can decrease their own contribution (the differential allocation hypothesis [DAH]). We investigated these hypotheses in the penduline tit Remiz pendulinus, a small passerine bird that has sequential polygamy by both sexes and strict uniparental care either by the male or the female. We focused on two sexually selected male traits: nest size and nest-building behavior. We show that male care is unrelated to nest-building behavior, whereas females are more likely to care for the offspring of those males that spend more time nest building. Females also more likely care for the offspring of males that build large nests. Consequently, the reproductive success of males increases with nest size and nest-building behavior. Our results are consistent with the DAH and suggest that nest-building behavior and nest size are under postmating sexual selection in penduline tits.