http://www.economist.com/news/science-and-technology/21719776-new-evolutionary-pressure-may-be-work-avian-world-small-brained-birds

"The smaller a bird's brain, when controlled for its body size, the more likely it was to have been road-kill."

"All this suggests a selective pressure on birds in parts of the world with lots of traffic to acquire bigger brains, even at the cost of the extra energy required to keep those brains airborne."