Each bird pair will have about 3–4 nestlings per brood, a clutch size of 4 has been identified as the most common and most successful. The cliff swallows brood-parasitize neighboring nests, where the females may move their eggs into or lay their eggs in other nests. The females who show intra-specific parasitism tend to have greater reproductive success than those that were brood-parasitized. The “victims” of brood-parasitism must nurture more chicks; with higher energy costs and decreased fitness because they are raising young that will not pass on their own genetic material. The male cliff swallows will also take part in this gene-spread by mating with more than just one female, contributing to genetic variation throughout the colony.