Cavity supply limits breeding for secondary cavity nesters, and the house wren's building behaviour reads as competition for that supply. The male stuffs several holes with coarse twigs, sometimes filling a box almost completely. The female then chooses one, removes or rearranges material and lines the cup with fine grass, feathers and hair.
The surplus structures have been given several names, including dummy nests and cock nests. They plausibly serve more than one function at once: advertising the male's territory quality to a prospecting female, denying cavities to competitors, and providing alternatives if a nest fails. These explanations are not exclusive and the evidence does not cleanly separate them.
House wrens are also unusually aggressive toward other cavity nesters. They are known to enter neighbouring nests and puncture eggs, including those of other species such as bluebirds and chickadees. It is an uncomfortable fact about a bird that nest-box guides tend to describe in affectionate terms.