In socially monogamous birds, levels of extra-pair paternity vary widely across species. A possible explanation for this variation is that the costs to both sexes of pursuing extra-pair copulations (EPCs) varies widely, which may substantially depend upon the effectiveness of the paternity assurance tactics employed by pair males. Given the prevalence of extra-pair paternity, an enigma exists in non-mate-guarding species in which males have no apparent means of assuring their paternity and yet rarely or never lose paternity. We propose that males in some species may pursue a subtle but effective paternity assurance tactic consisting of unpredictable behaviour. By departing and returning to and from their mates at unpredictable intervals males may be aisle to deter other males and their mates from attempting EPCs. Consistent with this idea, we found in the monogamous kittiwake Rissa tridactyla that the absences of males became decreasingly predictable in duration as their mates' fertile periods approached. Moreover, the durations of 80% of male absences were shorter than the minimum duration of copulations, creating a high probability of pair males returning in time to prevent EPCs. Factors other than male unredictability may explain the lack of extra-pair paternity in kittiwakes. Nevertheless, the unpredictable male hypothesis is testable in the many species with variable levels of EPP. Furthermore, we suggest that unpredictability may comprise a general strategy by which animals of both sexes protect various reproductive assets such as food, mates and territories.