Effects of Vehicle Speed on Flight Initiation by Turkey Vultures: Implications for Bird-Vehicle Collisions

Abstract

The avoidance of motorized vehicles is a common challenge for birds in the modern world. Birds appear to rely on antipredator behaviors to avoid vehicles, but modern vehicles (automobiles and aircraft) are faster than natural predators. Thus, birds may be relatively ill-equipped, in terms of sensory capabilities and behaviors, to avoid vehicles. We examined the idea that birds may be unable to accurately assess particularly high speeds of approaching vehicles, which could contribute to miscalculations in avoidance behaviors and ultimately cause collisions. We baited turkey vultures (Cathartes aura) to roads with animal carcasses and measured flight initiation distance and effective time-to-collision in response to a truck driving directly towards vultures from a starting distance of 1.13 km and at one of three speeds: 30, 60, or 90 kph (no vultures were struck). Flight initiation distance of vultures increased by a factor of 1.85 as speed increased from 30 to 90 kph. However, for 90-kph approaches there was no clear trend in flight initiation distance across replicates: birds appeared equally likely to initiate escape behavior at 40 m as at 220 m. Time-to-collision decreased by a factor of 0.62 with approach speeds from 30 to 90 kph. Also, at 90 kph, four vehicle approaches (17%) resulted in near collisions with vultures (time-to-collision ≤1.7 s), compared to none during 60 kph approaches and one during 30 kph approaches (4%). Our findings suggest that antipredator behaviors in turkey vultures, particularly stimulus processing and response, might not be well tuned to vehicles approaching at speeds ≥90 kph. The possible inability of turkey vultures to react appropriately to high-speed vehicles could be common among birds, and might represent an important determinant of bird-vehicle collisions.

The effects of radar on avian behavior: Implications for wildlife management at airports

“…Birds would have from 0.3 to less than 0.1 s to make escape maneuvers in those portions of the flight, which may not be enough time to allow them a successful avoidance. There is some evidence that birds increase flight initiation distances with increases in vehicle speed, contrary to our assumption (Legagneux and Ducatez, 2013;DeVault et al, 2014), so our estimates of how much earlier birds respond to aircraft with radar in flight may be conservative. How birds might respond to radar at higher speeds, similar to aircraft in flight, is difficult to determine, and requires further study.…”

“…There have been many studies showing that animals can evaluate threats and modify flight initiation distance accordingly (reviewed by Stankowich and Blumstein, 2005). Cowbirds and white-tailed deer have also been shown to modify behavioral response times in response to properties of vehicle approaches similar to the one used in this study (Blackwell and Bernhardt, 2004;DeVault et al, 2014). If the radar treatment enhances the perceived risk of the approaching vehicle, this could lead to an earlier escape (Ydenberg and Dill, 1986;Cooper and Blumstein, 2013).…”

Speed kills: ineffective avian escape responses to oncoming vehicles

“…In instances where an individual showed no alert behaviour, we scored TTC alert as equivalent to TTC flight . We converted TTC alert and TTC flight to AD and FID, respectively, by incorporating vehicle speed using the following equations: [11]. In addition to becoming alert to vehicle approach and initiating the escape response, a bird must respond with enough time to cover the distance necessary to avoid the oncoming vehicle.…”

“…A dynamic spatial margin of safety is apparent when the animal allows a vehicle to approach to a fixed percentage of the alert distance (AD) before initiating escape [24]. Based on findings by DeVault et al [11], we expected that cowbirds would use a spatial decision rule, at least at the lower vehicle speeds.…”

“…For instance, Legagneux & Ducatez [19] demonstrated that several species of birds escaped earlier from oncoming vehicles as the posted speed limit increased, but the actual speed of vehicles had no effect on escape behaviours. DeVault et al [11] found that near-collisions with turkey vultures (Cathartes aura) increased with vehicle speed, suggesting that animals may have difficulty assessing the threat from high-speed vehicles. Another recent study, employing vehicle approaches at night, found that white-tailed deer (Odocoileus virginianus) did not adjust escape responses across vehicle speeds [20].…”