Determinants of traffic mortality of barn owls (Tyto alba) in friesland, the netherlands

Abstract

Collisions with vehicles can contribute significantly to overall mortality in many species. We aim to clarify causal factors affecting Barn Owl (Tyto alba) mortality along roads in the Province of Friesland, The Netherlands. Although Barn Owl road casualties have received a lot of attention in the past, relationships with food availability and road design have not been clarified. We quantified breeding densities and reproduction rates, and combined these with road casualty records and characterization of road design. We obtained 996 ring-recoveries of Barn Owls (1994–2009), all of which were found dead in Friesland. All locations were accurate to within 100 meters. Along three routes with minor and major roads we made an inventory of road, verge, and hinterland typology. In two other datasets (539 and 461 birds) we analyzed the relationships between road mortality, body mass, and sex ratio. More than 70% of all owl casualties were in their first year of life and 15% in the second year. The highest mortality was in autumn and winter. Sex ratios were equal (232 males, 229 females). Although casualties were on average lighter than live-caught control birds, many of the traffic victims were in good condition. In years of high owl productivity, road mortality was strongly reduced. The number of road victims on major roads was significantly higher than on other road types. Highway junctions and highway-secondary road junctions contributed significantly to the spatial clustering of owl mortality. We conclude that in years when prey availability in the agricultural fields is high, roads/verges are not favored as hunting grounds. Therefore, the best option to reduce owl traffic victims is to improve the quality of farmland as foraging areas, and secondly to make design adjustments at major junctions to reduce owl-traffic collisions.