Breeding density and breeding synchrony have been reported to influence the intensity of sperm competition although results obtained so far are not conclusive. In this study we examine the investment of male lesser grey shrike (Lanius minor) into paternity guards in relation to breeding synchrony and breeding density. Male shrikes perform a double strategy to ensure paternity. They copulate frequently, mainly after territorial intrusions by other males, and guard their mates throughout the whole fertile phase. Our results indicate that, the two socio- ecological factors investigated, indirectly affect extra-pair behaviour in LGS’s. Males seem to be constrained by the frequency of intrusions by neighbouring males and this risk is associated with laying synchrony. The risk of intrusions depends on the timing and overlap of breeding attempts and males adjust their investment to paternity assurance accordingly. Furthermore, females seem to alter their egg laying patterns to minimise synchrony in situations where they find themselves in dense breeding situations. This is indicated by the negative correlation between breeding density and breeding synchronization. Extra-pair paternity, however, was not detected in our population. Thus the rate of extra-pair paternity is not necessarily an adequate measure to identify the influence of socio-ecological factors on male and female mating strategies.