Hundreds of millions of birds are killed every single year from being struck by cars. Now, it appears that those unfortunate enough to end up under the tire have something in common – it seems that only the bird-brained get hit.
Researchers looked into the brain size of bird species that get hit by cars and found that those with the smallest brains are most likely to be killed. They analyzed the remains of over 3,500 birds from 251 species killed on roads in the US, and found that while there was a significant difference in brain size, there were no differences in liver, heart, or lung mass.
Birds have been found to readily adapt to the challenges of living near roads. They can learn the speed limits, the differences in lane use, and adapt to the direction of traffic. It's possible, suggests the study published in Royal Society Open Science, that birds with the largest brains are better able to learn and adjust their behavior. Crows, for example, are well known for their intelligence, and will continue feeding on carrion even with traffic passing within inches. Only when a vehicle is coming right at them will they move, suggesting they are pretty good at judging behavior.
Interestingly, the researchers found that not only were the bird species that were most likely to be hit by cars those with the smallest amount of grey matter, but that this relationship held even within species studied. Again they found that, from sparrows to robins, the teeny-brained birds were the ones being killed most frequently.
This could mean that traffic incidents may be acting as a selection pressure, pushing the birds that frequent roads to evolve bigger brains. The researchers of the paper think, however, that this is probably unlikely, because cars have only really been that ubiquitous for the last half century or so, meaning that not enough time has passed for it to have had any significant impact on evolution.
But in saying that, there are examples where human influence, and roads specifically, may have caused a quantifiable change in the morphology of a species. Cliff swallows nest on rock faces high above the ground. In Nebraska, road bridges have filled this place – as such the swallows have suffered high mortality rates from being struck by cars. Yet over time, they have evolved shorter wings, something that could offer them superior maneuverability when darting away from cars.
This is unlikely to happen with all bird species, though, as the selection pressure is probably not strong enough across the board. Cats, however, are a different question entirely.