Males of several species of wrens (Troglodytidae) participate to varying degrees in nest building, some even building multiple nests (Kendeigh 1941, Verner 1965, Collias and Collias 1984, Kennedy and White 1992). The Northern House Wren (Troglodytes aedon) is a monomorphic, insectivorous, secondary cavity nesting species which is seasonally monogamous although polygyny does occur (Kendeigh 1941, Drilling and Thompson 1988, Johnson and Kermott 1991). Male House Wrens build multiple nests within their territories by placing sticks into several cavities (Kendeigh 1941, McCabe 1965, Finch 1989), while one cavity is the focus of the male's attention and receives the most sticks (Kendeigh 1941, Belles-Isles and Picman 1986). Soon after a female arrives on the male's territory she constructs a soft nest of rootlets, grass, and feathers on top of the stick foundation into which she deposits her eggs (Kendeigh 1941, Kennedy and White 1992). Although females insert some sticks into cavities (Kendeigh 1941, McCabe 1965), it is the male that expends the most effort filling nesting cavities with sticks (Kendeigh 1941, Kennedy and White 1992). Two hypotheses advanced (Kendeigh 1941) to explain the function of this behavior were territorial claim and female choice. Since competition for suitable nesting sites may be high for cavity-nesting species (Yahner 1983/1984, Brawn and Balda 1988, Gustafsson 1988), by placing sticks into cavities first, early arriving males may outcompete later arrivals for a favorable nest site. In many monomorphic species, males may be subject to stronger sexual selection which may manifest itself in behavioral rather than morphological consequences (Andersson 1994). Females may therefore select males based on the extent to which the stick foundation is completed (Kendeigh 1941), and may evaluate a male's commitment to her and the nest site similar to the Black Wheatear, Oenanthe leucura (Moreno et al. 1994), perhaps reducing her chances of being a secondary female if mated with a polygynous male. The extent to which the sticks serve a specific function with regard to the nest structure itself has not been tested. By preventing males from filling treatment nest boxes with sticks, I tested two hypotheses. First, if filling a nest box with sticks by males is necessary for courtship and mating, then stick removal from boxes should preclude pair-bond formation. Second, if having a stick foundation enhances fledgling success in some way, than pairs with sticks removed should be less successful in rearing and fledging young.