Abstract
We found that bumble bees Bombus ardens Smith tried to breed in the nest boxes of Varied Tits Parus varius and Great Tits P. major by exploiting their nest materials. In almost all cases, bumble bees utilized completely built and /or egg-laid nests of the tits. It is possible that bumble bees displaced the tits from the nest boxes. However, they did not use the nest boxes of the tits in the area where a steep ridge blocked their flight way to their pollination sites. It is reasonable to assume that the ridge acts as a barrier to the bees collecting nectar, which discourages them from using the nest boxes in such a location. The nest materials of the tits would provide good insulation for bumble bees.
