Our view of avian mating systems has been revolutionised by the use of molecular tools that have provided evidence supporting theoretical predictions that extrapair paternity (EPP) and intra-specific brood parasitism (IBP) would be widely observed alternative mating strategies in socially monogamous species. Quasi-parasitism (QP) is a third type of alternative mating strategy, where a female lays an egg in another female's nest and that egg is fertilised by the male partner at the parasitised nest. In contrast to both EPP and IBP, QP has been reported in only 12 species to date. We explore reasons for the apparent rarity of QP in birds and conclude that it is only likely to be adaptive in a fairly restrictive set of circumstances. We also review all of the evidence for the occurrence of QP in birds and find that it is far more limited than generally believed, as many apparent examples may be explained by rapid mate--switching or errors in molecular analysis of parentage. We suggest a number of criteria that need to be met for an unequivocal demonstration that QP has actually occurred.