Flexible parental care in a songbird correlates with sex-specific responses to seasonal phenology, mating opportunity and reproductive success.

Species with diverse parental care systems exhibit flexibility in response to environmental changes. Such changes can influence parental care decisions by altering the trade-offs between current and future reproduction. While previous studies have revealed correlations between specific social and ecological factors and parental care systems, the joint effects of multiple factors could be complex and may result in different outcomes within and across populations and between sexes. In this study, we observed parental care systems and their seasonal variations across three geographic populations in a polygamous species, the Chinese penduline tits (Remiz consobrinus), by monitoring breeding events over 8 years. In this species, biparental care, female-only care, male-only care and biparental desertion coexist in one population. To investigate whether and how sex-specific parental care patterns correlate with local conditions, we compared breeding phenology, season length, mating opportunity and reproductive success and their associations with the frequency of parental care patterns observed within and across the populations. We found striking population differences in the parental care system: biparental care predominated in one population, whereas female-only care was more common in the other two. These divergent parental care systems were shaped by sex-specific responses to the interacting local conditions. In all three populations, females synchronised egg-laying and offered males a short but highly successful re-mating window. This resulted in rapidly decreased male mating opportunities due to male collective clutch desertion. Males were more likely to care when re-mating opportunities were low. In contrast, females consistently provided care, regardless of variations in female mating opportunities. Additionally, males deserted clutches more in populations where desertion resulted in no fitness cost to the current broods. We found that desertion costs on brood fitness matter more than achieving a high reproductive reward in driving uniparental care, contrary to the theory that uniparental care only occurs when single parents can raise young efficiently. Our study demonstrates that parental care system variance across populations could result from the combined influences of social and ecological conditions, and their interactions. We highlight the consistency of sex-specific responses to local conditions in parental care.