Barn Owls (Tyto alba) are frequent casualties of roadway mortality throughout their global range. One of the world’s highest rates occurs along Interstate 84 (I-84) in southern Idaho, USA. We were interested in the extent to which dead Barn Owls were locally produced or if they arrived from wider geographic extents, which is important for quantifying the spatial influence of roads. Thus, using analysis of feather hydrogen isotopes, we assessed the geographic origins of dead Barn Owls (n = 150) found along I-84 between 2004 and 2021. We focused isotopic analysis on the first primary because in Barn Owls it may not be molted for four or more years from hatching; thus, even birds that dispersed to southern Idaho as juveniles would retain the isotopic signature of their natal area for years after arrival. Contour feathers from the breast or back of live nestlings (n = 50) captured from nests near the Interstate highway were used as a reference for comparison to dead birds. We reasoned that if dead birds were of local origin, both mean and variability of hydrogen isotope ratios would resemble those in locally produced nestlings. In contrast, if mean hydrogen isotope ratios differed, and/or if isotopic variability in road-killed owls was more extensive than in reference nestlings, we could infer that dead birds were from a broader geographic origin, and the variability would reflect the likely spatial extent. In this manner, the possibility that southern Idaho and the I-84 corridor attracts Barn Owls from wide geographic extents only for them to meet their demise along the interstate through vehicle collisions can be elucidated.