Аннотация:Extra-pair copulation (EPС) occurs in most socially monogamous bird species. The evolutionary, ecological, behavioural and physiological causes for the frequent occurrence of extra-pair offspring (EPO) in broods are still unclear. EPCs only occur when there is physical contact between individuals. Consequently, all factors that reduce encounters between individuals must affect the likelihood of extra-pair paternity (EPP). And if this is the case, then this effect should be apparent both in time (if there are significant changes in breeding density throughout years in a population) and in space (if there is any spatial heterogeneity in the number of contacts between individuals). Therefore, we investigated whether there is an effect of breeding density and location of the nest within a regular rectangular plot on the involvement of birds in EPC. Both factors affect the number of contacts with close neighbours. We checked paternity for six years (2005, 2013-2015, 2018, 2019) in Pied Flycatcher broods in Western Siberia. During these years, we controlled paternity in 370 nests. The breeding density of the species varied dramatically during this period. The mean distance to the nearest neighbour changed about two times. Changes in breeding density were related to changes in the rate of EPP. The proportion of nests with EPP and the ratio of EPO among nestlings change by a factor of five (0.235 - 0.048 and 0.104 - 0.019, respectively). Here we show that breeding density and nest location in the study area affect the EPP rate and the EPY ratio.