Variation in behavioral traits may be genetically or environmentally determined, or both. Previous studies on nest building behaviour in captive birds have proposed that nest building is mainly genetically determined. However, to settle this question, cross-fostering experiments in the wild has been recommended. The focus of the present study has been nest building behavior in blue tits (Cyanistes caeruleus) and great tits (Parus major) studied in a woodland near Oslo in Norway, where respectively, 95 and 79 nests of the two species were observed. Both build nests of mosses, lined with hair and wool. Blue tits also often apply feathers, which are only occasionally used by great tits. Some individuals of both species were interspecifically cross-fostered, i.e. blue tit nestlings were raised by great tit parents, and great tit nestlings were raised by blue tit parents. This provided the opportunity to study, in a natural environment, whether the interspecific variation in use of nest materials is primarily genetically or environmentally determined (due to learning from conspecifics, i.e. cultural transmission). The results show that, with regard to nest building, cross-fostered blue tits and great tits resemble members of their own species and not members of their foster species. This held true even when accounting for possible confounding variables such as laying date of first egg, clutch size, egg volume, and age. This implies that the variation in nest building behavior between these species is largely genetically determined. We suggest that nest building is a conservative trait that has evolved differently in blue tits and great tits.