Productivity, choice between colonial and solitary nest spacing and, mate fidelity were studied in the European tree sparrowPasser montanusL. in a suburban area of Budapest, Hungary. Nestboxes were arranged in five plots, in each plot, 25 were placed at 50 m intervals to simulate solitary breeding and 25 were placed 3-5 m apart to simulate colonial breeding. We recorded parental condition, number of eggs and fledglings, recruitment and life-span in the study area. Females raised most fledglings and recruiting offspring and reached their longest life span in the study area when they bred colonially for the first breeding year and they moved to solitary for all subsequent breeding seasons. The shift in nest spacing was influenced by low productivity in the colonial nests and the poor condition, as assessed by body mass, at the end of first breeding year. Both productivity and the condition of the females improved in the second breeding year if they took new partners which had greater previous breeding experience than themselves. Males produced most fledglings and recruiting offspring and reached their longest life-span if they bred in colonies throughout their lives in the study area. For all males, in all breding years, it was more advantageous to maintain the pair bond rather than to take a new partner. Males had a lower productivity and shorter life-span than did females.