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Co-operative breeders provide parental care to non-filial offspring—a behaviour known as ‘alloparental care’. While inclusive fitness benefits are a widely accepted driver of alloparental care in kin-based social groups, such indirect benefits are lost in non-kin societies. Among such societies, theory predicts that the degree of parental and alloparental effort should therefore be proportional to an individual’s genetic contribution to mixed broods—depending upon reproductive options. Using genotyping data across five to 12 microsatellite loci for individuals from 20 social groups (67 adults and 153 nestlings), we assessed whether kinship or proportional reproductive success explained trends in parental and alloparental effort in the Smooth-billed Ani (Crotophaga ani), a joint-nesting cuckoo species. Nocturnal incubation in this species appears to be performed almost exclusively by a single male. We first report significantly higher degrees of relatedness between adults within social groups (r¯ = 0.208, n = 114 dyads), than between social groups (r¯ = 0.120, n = 893 dyads), suggesting that inclusive fitness benefits may in part explain uneven allocation of parental effort. Second, we show that nocturnal incubation status is a significant predictor of reproductive success in males, as nocturnal incubators sire a greater proportion of nestlings in mixed-parentage broods. While patterns of reproductive skew appear high at 80% paternal confidence (B¯ = 0.052, p = 0.061), we report no significant deviation from an egalitarian breeding framework. Our results revealed similar patterns of reproductive allocation to closely related Groove-billed Anis (Crotophaga sulcirostris); however, differences in male reproductive skew and within-group relatedness across crotophagids are highlighted and offer insight into social evolution among anis. [ABSTRACT FROM AUTHOR]