Several comparative studies have previously identified breeding density and synchrony as potential determinants of reproductive success and extra-pair mating. However, the mechanisms and interaction of these two factors are poorly known. Here, we examined the effects of breeding density and synchrony on the behaviour, reproductive success and paternity losses in house sparrows. In order to test the effects of colony size, we created nest sites with varying numbers of nest-boxes. Our results show that there is an interaction between breeding synchrony and density, namely that breeding synchrony decreased with colony size. Neither colony size nor breeding synchrony seemed to influence brood size at fledging, although birds in larger colonies laid larger clutches. Moreover mate guarding behaviour was not influenced substantially by these two factors. Only nest guarding was significantly related to colony size and breeding synchrony. Paternity losses were not significantly related to colony size but they appeared to decrease with increasing synchrony. This finding supports the idea that extra-pair fertilisations are under male rather than female control.