Do roe deer react to wildlife warning reflectors? A test combining a controlled experiment with field observations
Abstract
Millions of animals are killed by vehicle collisions each year. As mitigation measures, wildlife warning reflectors have become increasingly popular, although clear evidence for their effectiveness is lacking. A reason for inconclusive results in the literature may be that most previous studies on the effectiveness of wildlife warning reflectors compare animal-vehicle collision rates with and without reflectors, a setting characterised by low event rates and weak experimental control. Animal behaviour can be expected to provide a more direct evidence for a possible effect of reflectors. In this study, we analyse roe deer behaviour in the presence of a blue semicircle reflector, one of the most frequently applied wildlife warning reflectors in Germany and other parts of Europe. Behavioural response to these reflectors (classified as no reaction, vigilance, short-distance flight and long-distance flight) was recorded both under controlled experimental conditions with captive roe deer and for free-ranging roe deer at road sections with traffic occurrence. We used generalised linear mixed models (GLMMs) to test if reflector presence induced threat-related behaviour (vigilance, flight) and movement away from the reflectors. We found no significant evidence that the light stimulus emitted by reflectors was perceived as a threat or induced evasive movement. We conclude that our study provides no evidence that blue semicircle reflectors induce behaviour in roe deer that seems suitable to reduce roe deer-vehicle collisions.
What the paper shows and why it matters
(AI-generated)
Wildlife warning reflectors are meant to throw a vehicle’s headlight back into the roadside, so that deer take it as a threat and stay away from the road. The idea is not unreasonable, since deer eyes are sensitive to blue and green light. Yet evidence that reflectors prevent collisions is inconclusive: most earlier studies compared collision counts before and after installation, where events are rare and control is weak, and a reanalysis of such studies (Brieger et al. 2016) found no significant reduction in collisions. The paper argues that future work should measure behaviour directly. The problem it takes on is whether the reflected light is in fact perceived as a threat and makes deer move away from the road.
The study finds no significant evidence that blue semicircle reflectors change roe deer behaviour in a way that could prevent collisions, so, given the study's power, any effect that exists must be rather small. Two tests carry this. In an experiment, 31 captive roe deer crossed a mock road where a headlight was switched on for a few seconds, with and without reflectors but without vehicles (1820 crossings). The light itself made reactions stronger, but adding reflectors did not. In a field study, thermal cameras recorded free-ranging deer meeting real vehicles at five road sections in Baden-Württemberg, before and after reflectors were mounted (1091 events). There was no evidence that reflectors increased vigilance or flight or made deer more likely to move back from the road. Only a non-significant tendency for the reflector effect to change with distance to the road appeared (interaction, p = 0.06), and the paper found no sign that deer then moved away from the road.
The paper concludes that if reflectors affect roe deer behaviour, the effect must be rather small, and doubts that they would reduce collisions substantially. It would not recommend investing in broad use unless new studies convincingly demonstrate their utility in the field, and thinks measures aimed at drivers, such as lower speed limits or driver warning systems, seem better suited. Reflectors are clearly discernible to drivers, which may change driver alertness, but the paper notes that studies of this effect are lacking. A later study on a railway line found no effect of reflectors on escape probability and cites the paper as consistent for blue reflectors (Jasińska et al. 2022).
References
- Brieger et al. (2016). Effectiveness of light-reflecting devices: A systematic reanalysis of animal-vehicle collision data. Accident Analysis & Prevention 97, 242–260. doi:10.1016/j.aap.2016.08.030
- Jasińska et al. (2022). A test of wildlife warning reflectors as a way to reduce risk of wildlife-train collisions. Nature Conservation 47, 303–316. doi:10.3897/natureconservation.47.73052