Death and Opportunity: The Hidden World Behind (Urban) Road Systems

Spotting roadkill while driving evokes sadness for many people. Imagine how intimidating our roads must feel for most animals: long concrete lines with strange metal boxes five times your size, moving faster than anything you have ever seen and producing terrifying amounts of noise. Roadkill is one of the leading causes of death for vertebrates (Schwartz et al., 2020). Yet this depressing scene of a carcass along the road symbolizes an end and a beginning, largely invisible to most people.

Why do cars collide with animals in the first place? Lima et al. (2015) found that behaviour towards approaching vehicles is species-specific and related to detection, threat assessment and/or evasive behaviour strategies (Figure 1). Many animals don’t recognize approaching cars as a natural threat, partly due to habituation. Combined with slow threat assessment, vehicle speed can overwhelm their sensory mechanisms. Certain evasive behaviours like freezing work well against motion sensitive predators. This will prove fatal against approaching cars, however, as many amphibians and reptiles have already realized too late.

Figure 1: key steps needed to avoid a collision with an approaching object. A collision happens if any step fails (‘N’). You only avoid a collision when all steps succeed (‘Y’). The curved arrows show where initially unsuccessful actions might be corrected with enough time (Lima et al., 2015).

These evolved strategies are essential for surviving in the natural world, but sometimes fail in the man-made world we constructed around them. Behaviour can also vary seasonally – wild boars (Sus scrofa) take greater risks alongside highways during periods of food scarcity in winter (Brieger et al., 2022). However, knowledge about vehicle collision behaviour across taxa remains limited (Lima et al., 2015), despite its significant impact on both humans and urban wildlife worldwide.

Road density significantly influences roadkill risk along urban-rural gradients, though this impact varies per species (Kent et al., 2021). For rural species like badgers, rabbits and pheasants, roadkill risk decreases beyond a road density threshold of 5000 m/km² (representative for villages and the outskirts of cities). In contrast, urbanized species like hedgehogs, foxes, pigeons and gulls face persistently high risks despite increasing road densities. These generalist species have successfully colonized urban environments and often thrive there in greater numbers than their rural counterparts.

However, while cities offer optimal niches for these generalist animals, they remain poorly equipped to handle the mortality risks posed by roads. This vulnerability likely stems from behavioural adaptations common in urban wildlife, including increased boldness and habituation to human presence, resulting in slower perception of threats and consequent responses as mentioned previously. Feeding strategies can also play a crucial role. Urban gulls, for instance, routinely scavenge roadkill (Figure 2), a behaviour hypothesized to increase their collision risk. This strategy has proven deadly, with urban gull roadkill rates exceeding what their population numbers would predict (Kent et al., 2021).

Figure 2: Lesser black backed gull (Larus fuscus) feasting on a road killed Chinese water deer in the parish of Broome, England (Pye, 2021).

Still, as can be noted from the gull example above, roadkill provides valuable feeding opportunities for urban scavengers. Schwartz et al. (2018) found seven species – herring gull (L. argentatus), lesser black-backed gull (L. fuscus), carrion crow (Corvus corone), Eurasian magpie (Pica pica), red fox (V. vulpes), domestic dog (Canis familiaris) and domestic cat (Felis catus) – scavenging on chicken heads representing roadkill corpses in the city of Cardiff, UK. These scavengers display distinct ecological niches, with gulls primarily operating in residential areas while corvids favour parks.

Temporal partitioning is also evident. Cats and foxes only remove carcasses during nighttime hours, while the rest primarily scavenge during daylight (Figure 3). Interestingly, there have been documented cases of nocturnal scavenging of gulls facilitated by artificial street lighting (Rock, 2005). This suggests that roadside illumination could likely create novel nocturnal feeding opportunities for traditionally diurnal species like gulls and corvids that would otherwise be unavailable in natural settings.

Figure 3: Frequency of scavenging in hourly periods for different species at camera traps baited with chicken heads (to simulate roadkill) within residential and parkland areas within the city of Cardiff, UK. Shaded areas represent times between sunset and sunrise (Schwartz et al., 2018).

Although urban scavengers are often seen as pests and persecuted as such, they provide a valuable hygiene-based ecosystem service by efficiently removing roadkill. Since both animal-vehicle collisions (Caro, Shargel & Stoner, 2000) and scavenging behaviour (Schwartz et al., 2018) typically occur at night and early in the morning, roadkill carcasses are removed relatively fast after collisions. Schwartz’s study (2018) demonstrated this rapid removal rate, with 76% of simulated roadkill (chicken heads) disappearing within just 12 hours. This mainly concerns smaller bodied animals that can be more easily carried away.

This efficient cleanup creates a significant challenge: roadkill surveys conducted after these crucial night and early morning hours substantially underestimate wildlife mortality from traffic. Such underestimations can compromise conservation planning and distort our understanding of urban species distributions based on these roadkill surveys. Therefore, to obtain more accurate assessments, roadkill surveys should ideally be conducted during early morning hours and incorporate adjustment factors based on known scavenger removal rates.

Rapid removal of roadkill by urban scavengers not only highlights the need for adjusted survey methodologies, but also underscores the broader scientific and ecological value of roadkill monitoring (Schwartz et al., 2020). When powered by citizen science projects, like iNaturalist, these observations can provide insights into population dynamics, behavioural adaptations, species distributions and environmental health. In Florida, a 99.3% reduction of raccoon roadkill observations highlighted the devastating effects invasive Burmese pythons (Python bivitattus) have on the local raccoon (Procyon lotor) populations (Dorcas et al., 2012). Additionally, roadkill specimens can serve as environmental sentinels to detect contaminants and disease vectors across numerous species. In this way, tragic wildlife losses can be transformed into valuable scientific data. In our rapidly changing urban landscapes, these datasets become increasingly vital for understanding biodiversity patterns, informing wildlife conservation decisions, and ultimately designing more wildlife-compatible infrastructure that reduces the very mortality it studies.

References

Brieger, F., Kämmerle, J. L., Hagen, R., & Suchant, R. (2022). Behavioural reactions to oncoming vehicles as a crucial aspect of wildlife-vehicle collision risk in three common wildlife species. Accident Analysis & Prevention, 168, 106564.

Caro, T. M., Shargel, J. A., & Stoner, C. J. (2000). Frequency of medium-sized mammal road kills in an agricultural landscape in California. The American Midland Naturalist, 144(2), 362-369.

Dorcas, M. E., Willson, J. D., Reed, R. N., Snow, R. W., Rochford, M. R., Miller, M. A., … & Hart, K. M. (2012). Severe mammal declines coincide with proliferation of invasive Burmese pythons in Everglades National Park. Proceedings of the National Academy of Sciences, 109(7), 2418-2422.

Kent, E., Schwartz, A. L., & Perkins, S. E. (2021). Life in the fast lane: roadkill risk along an urban–rural gradient. Journal of Urban Ecology, 7(1), juaa039.

Lima, S. L., Blackwell, B. F., DeVault, T. L., & Fernández‐Juricic, E. (2015). Animal reactions to oncoming vehicles: a conceptual review. Biological Reviews, 90(1), 60-76.

Pye, A. S. (2021). Lesser black back gull feasting on a dead Chinese water deer. Geograph. https://www.geograph.org.uk/photo/6896315

Schwartz, A. L., Shilling, F. M., & Perkins, S. E. (2020). The value of monitoring wildlife roadkill. European journal of wildlife research, 66(1), 18.

Schwartz, A. L., Williams, H. F., Chadwick, E., Thomas, R. J., & Perkins, S. E. (2018). Roadkill scavenging behaviour in an urban environment. Journal of Urban Ecology, 4(1), juy006.

Featured image by Chinmayisk