Over the course of this year, the motor vehicles using our roads will clock up some 300 billion miles of travel between them. With so much traffic on our road network, much of it moving at speed through rural areas, it is little wonder that wildlife casualties are a common sight; forlorn bundles that have been stripped of life and now lay in the gutter. The risk to our nocturnal wildlife is elevated as the autumn evenings begin to draw in and our evening commute home overlaps increasingly with the emergence of owls, deer and foxes.
The barn owl seems particularly susceptible to collision with motor vehicles and a great many, possibly 5,000 to 6,000 individuals, are killed on our roads each year. Much of this toll happens during the autumn, a period when young barn owls, newly independent and inexperienced, are moving away from their natal sites to set up home elsewhere. Over the course of three or four months they will cover a dozen or so kilometres. During this period they will, invariably, encounter a road. Whether or not they are then hit by a lorry or a car depends on a number of different factors. Vehicle speed and traffic volume are important but so are other things, such as whether or not the road is bordered by a hedgerow, whether it has a wide grassy verge and whether it is sunken or raised.
Various studies have revealed that a raised road, running across open country, poses a particular risk because an owl is likely to cross the road at bonnet level. Conversely, a sunken road is more likely to be crossed at a greater height, reducing the risk of collision. Hedgerows work in a similar manner, forcing the bird up and over the road. The presence of grassy verges can be a problem for a different reason. Such verges often support good numbers of field voles, a favoured prey species, and may actually attract owls to the road in those areas where other hunting opportunities are limited. Buffeted by the back draft from a passing lorry, the disoriented owl may then be pulled into the path of the next vehicle and hit.
While this knowledge may help us to plan the management of our roads and their boundary features better, reducing owl mortality, some of the solutions for owls may increase the risk of mortality to other wildlife groups. Replacing a grassy verge with densely-planted shrubs might stop an owl from hunting but it might increase thrush mortality, with birds attracted to the berries that such shrubs often carry. Mitigating the impacts of our busy road network needs careful thought and sensitive planning.