It's an all too familiar scene: you're driving along a smooth stretch of road, not an obstacle in sight, when, suddenly, a group of birds/frogs/lizards/furry animals materializes in your path. You try honking, flashing your lights, but to no avail. Finally, just when it seems like impact is inevitable, the critters scatter in all directions, leaving you with a clear road, but a fast-pumping heart. Although these road–wildlife interactions often end innocuously, animal–vehicle collisions are all too common in our busy world. But why do some individuals mis-time their escapes? Do animals adjust their response according to the speed of the approaching threat? Scientists from the US Department of Agriculture teamed up with researchers from Indiana State and Purdue University in Illinois, USA, to face these questions head on.
Travis DeVault and his colleagues suspected that animals make decisions about when to escape based on one of three types of behavioural rules: the distance between themselves and the threat (e.g. always escape when threat is 10 m away), the time between themselves and contact (e.g. always escape when a threat is 1.5 s away) or a dynamic integration of both distance and time. To figure out which of these rules applies when assessing vehicles approaching at different speeds, DeVault and his team had to be creative. They edited real video recordings of a car approaching head on to simulate eight driving speeds ranging from 60 to 360 km h−1. This range encompasses everything from average city driving to airplane take-off and landing speeds. Next, they played these edited videos back to groups of captive brown-headed cowbirds (Molothrus ater) and filmed their reactions. From observations and recordings of escape speeds in wild individuals, the team estimated that cowbirds need at least 0.8 s to clear the path of an oncoming vehicle to avoid a collision. Knowing this, they were able to assess whether the alert and flight initiation times and distances used by the birds enabled a successful escape at each simulated speed.
The researchers found that cowbirds use a fixed escape-distance rule when deciding when to flee. This strategy worked quite well when faced with vehicles moving at low speeds. When simulated approaches were less than 120 km h−1, most birds got out of the way in time. Unfortunately, the failure to adjust flight initiation distance according to the context was bad news for birds when speeds increased: with less time to impact once their distance threshold was passed, most individuals could not avoid a collision when speeds exceeded 120 km h−1.
Escape strategies based on distance rather than time are probably quite effective against natural threats like hawks, foxes and wolves, which approach at slower and more predictable speeds. However, the fast-paced world of planes, trains and automobiles may be too overwhelming for most wildlife to process. More research is needed to find viable solutions for preventing these road–wildlife interactions. Until then, be sure to buckle-up, keep your eyes on the road and SLOW DOWN!