Who takes care of the kids at when? Variation of sex-role patterns across different parental care forms in passerine birds

Parental care in birds usually consists of many elaborate forms, including nest building, incubation, provisioning the offspring and protecting them. Given the various life history differences between sexes, parents may have different opportunity costs of providing care in a specific form. But we still do not know whether males and females generally differ in their involvement in different care forms across stages of a breeding cycle, such as nest building, incubation and chick provisioning. Here, we performed a survey of parental care regarding which sex provides care in 882 species of passerine birds and found significant differences in the frequency distributions of sex-role patterns (i.e. female-only care, biparental care, and male-only care) across three distinct forms of parental care. This result showed clearly that parental care should not be treated as a unitary trait, but a composite of integrated features with diverse functions. Using a set of linear mixed-effect models we tested the effects of sexual selection, certainty of paternity, predation risk, and offspring9s life history traits in driving the variation in sex roles. In general, we found species with female-only care tended to be under strong sexual selection on males, and uncertainty of paternity could reduce male care.