Barn Owl Roadway Mortality: Understanding Why More Females Than Males are Involved

Wildlife-vehicle collisions (WVC) kill billions of animals every year. Among birds, WVC is particularly high for Barn Owls ( Tyto alba ) throughout their widespread range, especially during the nonbreeding season. One common pattern is that female Barn Owls outnumber males in WVC. For instance, Boves and Belthoff (2012) reported 1.4 times more roadkilled females than males in Idaho, and Moore and Mangel (1996) found dead females 2.8 times more frequently than males in California. Reasons for this disparity remain poorly understood. We evaluated the hypothesis that female-biased mortality is caused by wider-ranging behavior, more frequent road crossings, and greater proximity to roads by females. We tracked male and female Barn Owls with GPS data loggers during two winters (2019 and 2020) in southern Idaho, U.S.A., where high rates of road mortality are reported. Male Barn Owls traveled farther per night and crossed more roads than females and, when standardized to road crossings per km traveled, there was no difference in crossing rate between males and females. Finally, there was no difference in proximity to roads between males and females. Thus, there was no evidence that females encountered, crossed, or stayed in proximity to roads more than males. Indeed, males ranged more widely and crossed more roads per night than females. Thus, we are able to reject the hypothesis that female-biased road mortality is related to higher rates of road crossings for females. Instead, it is possible that when females do cross roads they are more susceptible to WVC because of their larger size and/or lower agility than males. Or perhaps higher numbers of WVC in females reflect a population sex ratio biased toward females.