Genetic monogamy in birds is the exception rather than the rule, despite social monogamy being the dominant avian mating system. Mating with males other than the female’s social male has been found in over 70% of the studied bird species, but despite 30 years of research, the enormous amount of variation in the occurrence and levels of extra pair fertilization (EPF) rates both within and between species remains largely unexplained. Research attempting to explain this variation has focused on a wide range of factors including the role of life-history and ecology. However, few general patterns have been identified other than that over 50% of the inter specific variation in EPF rates can be attributed to phylogeny occurring at or above the family level. This means that lots of the variation between species has been determined in the ancient avian lineage and that ecological patterns in the variation in EPF's might be due to phylogeny. Here we test the key hypotheses that have been proposed as explanations for variation in EPF's in the past 30 years. Using data on five species from 12 populations within the Malurus (fairy-wrens) genus, we test for the role of breeding density, inbreeding avoidance, fertility insurance, life-history and ecology on EPF's. Findings of this study will allow us to identify the general patterns in EPF rates within one genus, which could therefore not be confounded by phylogeny and should help us to understand the enormous amount of variation of this extraordinary behavior in other species.