Repeatability of an extended phenotype: causes and consequences of nest variation in a free-living songbird

Construction behaviour is an aspect of the extended phenotype that allows organisms to build structures that alter their environments in potentially beneficial ways. Although individuals vary in the expression of this extended phenotype (e.g., structure morphology), the repeatability of construction behaviour remains understudied, especially among free-living populations. Many oviparous taxa construct nests, making them of particular interest because variation in nest architecture may directly affect fitness. Using a free-living, cavity-nesting songbird, the northern house wren ( Troglodytes aedon aedon ), as our model, we estimated the contribution of the primary builder (the female) to nest variability by measuring the repeatability of nest morphology between successive clutches. We further examined whether nest morphology was related to the dimensions of the nesting cavity, breeding date, or nest success. We found the composition of the cup lining to be a highly repeatable behaviour for the nesting female, although the size and composition of the structural platform appeared more related to the dimensions of the cavity. Despite the expectation that construction effort would show a seasonal decline, similar to clutch size in this species, nest morphology remained incredibly variable throughout the breeding season and was unrelated to the survival of the offspring. Our study suggests that variation in construction behaviours is a product of multiple factors including the preferences of the builder and physical constraints. The absence of any clear links between construction behaviour and fitness indicates that nest morphology is not under strong selection. As a result, diverse female building preferences may explain the extreme among-individual variation in nest structure in this species.