A 2005 study by the U.S. Dept. of Agriculture Forest Service revealed that over 80 million birds are killed each year due to collisions with motor vehicles. However, one smart bird species that resides in some areas of the Midwestern United States like Nebraska, seems to be bucking the trend and if two researchers are to be believed, it is all to do with evolution.

The 30-year study led by University Of Tulsa's Charles Brown and his wife Mary Bomberger Brown, involved observing Cliff Swallows that reside in colonies under highway bridges, overpasses and road culverts. Like many animals that lack teeth, they are constantly on the roads picking up tiny bits of gravel that they swallow. These cement bits act like teeth inside their gizzard and assist in breaking down and digesting hard foods like seeds.

This together with the fact that they are aerial feeders who pursue their insect preys at speeds exceeding 20-25mph, often puts them right in the line of the ongoing car traffic that usually results in their deaths.

When the researchers began their study in 1982, about 20 Swallows perished annually from car collisions. By 2012, the numbers had been reduced to between just 2-4. Deducing that there was something more than luck involved, they began investigating other factors.

One of the things they measured was the wingspan of the birds had been killed over the years, the carcasses of which had fortunately been preserved. What they noticed was that over the years, the average wing length of most Swallows has been reduced from 110 mm (4.3 inches) to 103 or 104 mm (4.1 inches). What they also observed is that the ones that still sported longer wingspans became easier victims, possibly because they took longer to take off and gain altitude when confronted by the incoming car traffic.

The researchers who published the results of the study in the March 18th issue of Current Biology believe that while some Swallows still have longer wingspans making them susceptible to urban traffic, most have evolved, which is the reason they are thriving in a habitat where traffic patterns can only get worse.

Resources: usatoday.com,latimes.com, gizmag.com