Conspecific brood parasitism (CBP) is a reproductive tactic in which parasitic females lay eggs in nests of other females of the same species that then raise the joint brood. Parasites benefit by increased reproduction, without costs of parental care for the parasitic eggs. CBP occurs in many egg-laying animals, among birds most often in species with large clutches and self-feeding young: two major factors facilitating successful parasitism. CBP is particularly common in waterfowl (Anatidae), a group with female-biased natal philopatry and locally related females. Theory suggests that relatedness between host and parasite can lead to inclusive fitness benefits for both, but if host costs are high, parasites should instead target unrelated females. Pairwise relatedness (r) in host–parasite (h-p) pairs of females has been estimated using molecular genetic methods in seven waterfowl (10 studies). In many h-p pairs, the two females were unrelated (with low r, near the local population mean). However, close relatives (r = 0.5) were over-represented in h-p pairs, which in all 10 studies had higher mean relatedness than other females. In one species where this was studied, h-p relatedness was higher than between nesting close neighbours, and hosts parasitized by non-relatives aggressively rejected other females. In another species, birth nest-mates (mother–daughters, sisters) associated in the breeding area as adults, and became h-p pairs more often than expected by chance. These and other results point to recognition of birth nest-mates and perhaps other close relatives. For small to medium host clutch sizes, addition of a few parasitic eggs need not reduce host offspring success. Estimates in two species suggest that hosts can then gain inclusive fitness if parasitized by relatives. Other evidence of female cooperation is incubation by old eider Somateria mollissima females of clutches laid by their relatives, and merging and joint care of broods of young. Merging females tended to be more closely related. Eiders associate with kin in many situations, and in some geese and swans, related females may associate over many years. Recent genetic evidence shows that also New World quails (Odontophoridae) have female-biased natal philopatry, CBP and brood merging, inviting further study and comparison with waterfowl. Kin-related parasitism also occurs in some insects, with revealing parallels and differences compared to birds. In hemipteran bugs, receiving extra eggs is beneficial for hosts by diluting offspring predation. In eggplant lace bugs Gargaphia solani, host and parasite are closely related, and kin selection favours egg donation to related females. Further studies of kinship in CBP, brood merging and other contexts can test if some of these species are socially more advanced than presently known.