There is growing evidence, suggesting that artificial light sources have negative impact on a large number of organisms. Nevertheless, light pollution is a relatively new term, since artificial light affects the environment in more subtle ways than any other anthropogenic factors, e.g. habitat destruction or noise.  Birds are important model species in regard to this, since their behaviour and reproductive cycle is known to be highly sensitive towards light conditions. To investigate how light-at-night affects the reproductive performance of passerines, we conducted an experiment on a population of wild living blue tits (Cyanistes caeruleus ). We combined results of a correlative long term study with an experimental approach. Therefore we established artificial light sources at nestboxes during the nest-building period and compared breeding phenology and nestling condition between (i) natural habitats, (ii) experimental groups and (iii) street-lamp territories.  Our results reveal that birds choosing to breed around street lamps are not affected in breeding performance, whereas birds âforcedâ to breed under changed ambient light conditions produce nestlings with lower body condition in comparison to natural habitats.  Our results support the idea that artificial light sources affect the breeding performance of birds and bring additional knowledge important for conservation.