Extrapair paternity (EPP) is common in many socially monogamous species, influencing patterns of sexual selection and shaping many aspects of reproductive behaviour. However, factors explaining variation in the occurrence of EPP, both within and between populations, remain poorly understood. One ecological factor that has received considerable attention is breeding synchrony, but the proposed mechanisms remain contentious and the findings from the large number of correlational studies have been inconsistent. Mate guarding, a behavioural tactic to limit paternity loss, may be fundamental to any relationship between EPP and breeding synchrony. However, few studies have investigated how guarding behaviour varies with breeding synchrony, and the theoretical predictions are unclear. We examined how mate-guarding intensity in the colonial fairy martin varied with changes in breeding synchrony. To eliminate likely confounding effects of individual quality, we measured guarding intensity on multiple days during the fertile period of individual females and related this to daily variation in colony-level breeding synchrony. Similarly, we examined whether extrapair interest in fertile females varied with change in breeding synchrony. Both mate-guarding intensity and extrapair pursuit rate increased sharply several days prior to egg laying, before declining once laying commenced. When we controlled for this effect of female fertility status, guarding intensity increased with breeding synchrony. These novel findings suggest that the risk of paternity loss increases with breeding synchrony, at least among colonial species. Moreover, adjustment of guarding intensity to the risk of paternity loss may explain why most correlational studies do not reveal a relationship between EPP and breeding synchrony.