Density-dependence of extra-pair copulation rates and mate guarding intensity in the monogamous swallow Hirundo rustica was studied from 1983 to 1988 in N. Jutland, Denmark. Changes in local population size paralleled changes in national population size from 1976 to 1988. Median and mean colony size increased and minimum, mean and median nearest neighbour distance decreased with increasing population density. The percentage of female swallows involved in extra-pair copulations increased significantly with increasing population density between years and with increasing colony size within years in spite of the fact that male mate guarding intensity changed similarly. Mate guarding did reduce the probability of the female being involved in extra-pair copulations, but not to the extent that it compensated for the density-dependent increase in extra-pair copulation attempt rates. Natural and sexual selection arising from extra-pair copulations is frequency-dependent and varies spatially and temporally in relation to population density.