Assessment of Potential Effects of Roads and Land Cover on Brood Sex Ratios in Barn Owls in Southwestern Idaho

Additional Funding Sources

This research, conducted at the Raptor Research Experiences for Undergraduates site, was supported by the National Science Foundation and Department of Defense under Grant No. DBI-1852133 and by Boise State University.

Abstract

Wildlife-vehicle collisions (WVC) kill billions of vertebrates and invertebrates every year. Among birds, rates of WVC are particularly high for Barn Owls (Tyto alba), and one of the world’s highest roadway mortality rates for Barn Owls occurs along Interstate 84 (I-84) in southern Idaho, USA. Studies frequently find juvenile Barn Owls are killed through WVC in greater numbers than adults, possibly because juveniles outnumber adults and are simply killed in proportion to their abundance. Another common pattern is that WVC involve female Barn Owls more frequently than males. For instance, 1.4 times more females than males are killed along roads in Idaho (Belthoff and Boves 2012) and 2.8 times more frequently than males in California (Moore and Mangel 1996). A potential explanation for this pattern is that both males and females are vulnerable to road mortality, but females outnumber males on the landscape and are killed in proportion to their abundance. More females than males on the landscape could arise if offspring sex ratios are not 1:1, i.e., brood sex ratios are biased toward females. Our objective was to examine relationships between roads and other land cover on brood sex ratios in Barn Owls to examine the hypothesis that nests experiencing the greatest road intensity would be most likely to be female biased. To do so we used DNA isolated from owl blood to sex 726 nestlings from 167 broods during 2019-2021. Using a series of generalized linear mixed models, we then examined the potential influence of roads and other land cover covariates on brood sex ratios and examined spatial variation in the likelihood of male- and female-biased broods.

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Assessment of Potential Effects of Roads and Land Cover on Brood Sex Ratios in Barn Owls in Southwestern Idaho

Wildlife-vehicle collisions (WVC) kill billions of vertebrates and invertebrates every year. Among birds, rates of WVC are particularly high for Barn Owls (Tyto alba), and one of the world’s highest roadway mortality rates for Barn Owls occurs along Interstate 84 (I-84) in southern Idaho, USA. Studies frequently find juvenile Barn Owls are killed through WVC in greater numbers than adults, possibly because juveniles outnumber adults and are simply killed in proportion to their abundance. Another common pattern is that WVC involve female Barn Owls more frequently than males. For instance, 1.4 times more females than males are killed along roads in Idaho (Belthoff and Boves 2012) and 2.8 times more frequently than males in California (Moore and Mangel 1996). A potential explanation for this pattern is that both males and females are vulnerable to road mortality, but females outnumber males on the landscape and are killed in proportion to their abundance. More females than males on the landscape could arise if offspring sex ratios are not 1:1, i.e., brood sex ratios are biased toward females. Our objective was to examine relationships between roads and other land cover on brood sex ratios in Barn Owls to examine the hypothesis that nests experiencing the greatest road intensity would be most likely to be female biased. To do so we used DNA isolated from owl blood to sex 726 nestlings from 167 broods during 2019-2021. Using a series of generalized linear mixed models, we then examined the potential influence of roads and other land cover covariates on brood sex ratios and examined spatial variation in the likelihood of male- and female-biased broods.