Road kills a driving factor of natural selection among cliff swallows

Evolution is one of the most intricate objects of scientific investigations. Since Charles Darwin introduced the idea of natural selection in the 19th century, a wide range of related issues was raised and discussed. Till nowadays zoologists find more evidences of species evolutionary development, their connection to prehistoric creatures and factors that could intensify the process. 

A species of swallow in the US has evolved to have a
shorter wingspan [Credit: Getty Images]

One of the researches, recently conducted by Charles R. Brown from the Department of Biological Sciences, University of Tulsa, has contributed to the list of possible evolution factors. More precisely, the professor and his team emphasized that vehicular traffic may be a potential cause of natural selection, which they proved in case of birds’ evolution.

It took Charles R. Brown almost 30 years of field work at university biological center in Nebraska to make an astonishing conclusion. Specifically, it is a place of massive nesting of swallows, which, by the way, is located in a close neighborhood to highways and other road infrastructure. 

Every dead bird was recorded and investigated concerning the assignment of cause of death. It turned out that hundreds of birds were knocked down by cars or windshields. However, the statistics data marked a certain discrepancy that advanced over three decades: as the vehicle density increased, the number of dead swallows reduced. The possibility of birds’ evolution received substantial grounds.

The next step made by Charles R. Brown was related to the search of some visible similarities among wrecked bird individuals. The analysis showed that wings of the white-fronted swallows have become shorter over the past decade. Individuals killed in a collision with a car differed with much longer wings than the average in the population, and this difference got bigger from year to year. 

“There were fewer road kills, and the birds found along highways had longer wing spans,”
Brown said. “I wanted to know if there was selection for particular characteristics in
those dead birds.” [Credit: The University of Tulsa]

Precisely, this tendency appears to be a reflection of natural selection process, as birds with shorter wings are more likely to survive on the roads, and, besides, growth of their number in overall population is a consequence of birds’ adaptation to local environment.

The professor has revealed one more interesting scientific discrepancy: more and more swallows nest near dense roadways. It is clear as a day that viaducts, culverts, bridges and other transportation infrastructure provide some protection from foul weather, outranging any dangers associated with living near racing vehicles. 

Especially since transport as a risk factor loses importance gradually thanks to the shorter wings. Above all, the research made by Charles R. Brown has demonstrated that evolutionary factors might affect in short terms (in this case 30 years happened to be enough time to determine adaptive changes). 

As a result, one may observe Darwin’s Theory of Evolution at work: the example shows that swallows with shorter wings have more chances for survival and prevailing in the population. 

The study has been published online in the journal Current Biology.

Guest Post by Maria Kruk, an author for Species.com [April 05, 2013]