Wrens’ nesting habits differ from some other species in that they are not particular as to where they nest. They are opportunistic and tend to use a bird house much more readily than other birds.
Wrens are also more tolerant of people and can nest in odd places such as flower pots, mailboxes, hanging baskets, fixtures or garages. They are fiercely competitive and very territorial when choosing a nesting site and often harass and peck at much larger birds. They may even go so far as to drag the young birds out of an existing nest site. This behavior has contributed to the nest failure of bluebirds, tree swallows and chickadees.
Unlike other birds, when a male wren finds a suitable nesting spot, he places twigs and sticks to begin the nesting process before he has found a mate. He may begin the construction of many nests in various locations, often referred to as dummy nests, because ultimately it is the female who chooses only one. When he has started nest building in all of his sites, he attempts to find a suitable mate. The female inspects each site, even those with incomplete nests. When she approves a nest spot, she continues the building process, adding feathers, grasses, hair or any other appropriate soft material. If she finds materials the male has placed are not to her liking, she removes them, dismantling the nest one stick at a time.
Generally, house and Carolina wrens prefer natural cavities that are shaped so the nest is concealed. However, both use nest boxes. Some examples of unconventional nest sites chosen by wrens are scarecrows, tin cans, boots, cow skull and even pockets in hanging laundry.