Recent research into the Large-billed Gerygone, a host species of the Little Bronze-Cuckoo, has revealed that the host physically ejects parasitic cuckoo young from its nest, a behaviour not previously observed in any other host-brood parasite system. Curiously, this host does not reject dissimilar foreign eggs despite egg rejection seeming to be a better strategy for it because if successful, there is no risk of the warbler's own eggs being ejected by cuckoo young. In order to explain this puzzle, we present a new hypothesis, termed the egg dilution effect, which argues that parasitic cuckoo eggs serve to ensure host egg survival through the dilution effect and protect against parasitism by multiple females. It is therefore beneficial for hosts to accept cuckoo eggs even if they are capable of discriminating cuckoo eggs from their own. The conditions of this hypothesis, such as small clutch size and high parasitism rate, fit the nature of the host species and thus help explain why this unique anti-parasitic strategy has evolved only in the Large-billed Gerygone.