Hundreds of millions of birds, reptiles, and mammals die in animal-vehicle collisions around the world every year, yet we know very little about what actually goes wrong when they fail to escape. Why do deer caught in the headlights freeze and why do turtles retreat into their shells while they’re still on the road? Now, researchers studying the avoidance behaviors of birds reveal that their natural predator evading strategies are maladaptive for avoiding evolutionary novelties like our cars and aircrafts. The findings were published in Proceedings of the Royal Society B this week.
When animals are approached by predators, as well as non-biological threats, they must decide if and when to initiate their escape response. That decision is based on various characteristics of the oncoming object (size and speed, for example) as well as their own experiences, risk-taking behaviors, hunger level, how threatened they feel, their species’ anti-predation repertoire, and the costs and benefits of precisely timed fleeing. In other words, just getting out of the way isn’t as simple as it sounds.
Using video playbacks, a team led by Travis DeVault from the U.S. Department of Agriculture’s National Wildlife Research Center investigated the alert and escape behaviors of captive male brown-headed cowbirds (Molothrus ater) at NASA’s Plum Brook Station in Ohio. The virtual vehicles that approached the birds varied in size and speed, which ranged from 60 kilometers to 360 kilometers an hour. The researchers previously recorded video of a 2003 Ford F250 pickup truck driving on a flat road, and then edited the images accordingly. (They turned to virtual reality because simulating the high-speeds of cars on highways or planes during take-off and landing would likely result in real collisions.) Then, as the birds watched the videos in a custom-built chamber, the team recorded behavioral metrics including their alert time and flight initiation time.
Escape behaviors of the cowbirds, they found, appear to be based on the distance from the truck rather than the time available for escape. The cowbirds didn’t initiate escape in time to avoid collision when vehicle speeds exceeded 120 kilometers an hour, which didn’t give them enough time to assess the approaching vehicle.
While their decision-making process might be very effective for evading predators like hawks in the wild, the strategy might be maladaptive (and fatal) for avoiding modern vehicles—which differ from predators in many important ways, the least of which are speed, size, and the directness and consistency of approach.