Field experiments with nest boxes show that nest holes are a limiting factor on the local population size of the Collared Flycatcher Ficedula albicollis. When this limiting resource occurs with different types that are quality related there is intraspecific competition among males for the best nest boxes, with bigger males being the better competitors. Population dynamics of Great Tit Parus major and Blue Tit P. caeruleus when compared with Collared Flycatchers suggests that there is strong interspecific Competition for available nest holes. This is also shown by removal experiments.