Understanding the mechanisms behind road effects : linking occurrence with road mortality in owls

Several studies suggest that species’ mortality rates are positively related with local population abundances. Because owls have shown both high road mortality rates and road avoidance behaviours, we hypothesize that road-kill likelihood is not always directly linked to their occurrence. In this study, we examined the relationship between the likelihood of species occurrence in the vicinity of major roads and the road-kill risk for barn owls Tyto alba, tawny owls Strix aluco and little owls Athene noctua. Specifically, we address: (1) the role of road-related features on spatial patterns of species’ occurrence and road kills; (2) the composition of road kills and their distribution throughout the year; (3) the relationship between species’ occurrence likelihood and mortality risk. Our findings show that changes in movement patterns is probably the main behavioural mechanism that threatens owls in roaded landscapes. The high mortality risk of barn owls in autumn and winter seasons, after the peak of dispersal period, provides support for the hypotheses of expanding movements because of lack of food. However, mortality because of high occurrence likelihood seems to also explain tawny owls’ response towards roads. The high occurrence likelihood of little owls combined with low mortality rate also suggests avoidance of road crossings. Although, it is clear that within species there is variation according to age- and territory-holding status, closely related species have different sensitivities to roads as a function of their varying food preferences. We believe that linking species distribution with mortality risk may be more effective in focus conservation efforts. Therefore, we suggest measures should be applied to reduce prey availability close to roads and in road verges. This is particularly important for barn owls, for which foraging in the vicinity of roads during the cold season is especially risky.