Nest predation is a driving force in nest site selection and reproductive success across open-cup and cavity-nesting birds. However, we are just beginning to understand how perceived predation risk might affect nest site selection, particularly in cavity-nesting species. Because passerines have been shown to have a keener sense of smell than has traditionally been attributed to them, we tested whether Tree Swallows (Tachycineta bicolor) avoid nest cavities containing the odors of a potential nest predator, the house mouse (Mus musculus). Although Tree Swallows have historically suffered nest predation from deer mice (Peromyscus sp.), we hypothesized that these birds would equate these rodent odors. In the 2021 breeding season, we presented Tree Swallows with a choice of 2 nest boxes, 1 containing clean (unused) mouse cage bedding and the other containing bedding soiled with house mouse urine and feces. We concealed the bedding in both treatments under cardboard disks to eliminate visual cues and force Tree Swallows to rely on their olfactory abilities. We hypothesized that Tree Swallows would prefer to use odor-free boxes. Because Tree Swallows are constrained in their nest site choice, we did not assume that they would entirely avoid boxes with urine. Our analysis of the first box chosen from 46 box pairs in which Tree Swallows occupied both boxes found that nest boxes containing mouse odors were chosen first at the same frequency as boxes containing clean odor-free mouse bedding. However, we did find that the swallows showed a preference for wooden boxes.

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