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Evolution in action: Roadkill breeds birds with shorter wings


March 19, 2013

American cliff swallows building nests under a bridge

American cliff swallows building nests under a bridge

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The American cliff swallow is best known for its yearly migration between North and South America, traditionally resulting in the annual return of the swallows to Mission San Juan Capistrano in Southern California on March 19. Now it seems they also provide a lesson in the workings of natural selection. A three-decade long study carried out by a husband and wife ornithological team in western Nebraska has, thanks to long years of carefully recording all available data, shown that roadkill has exerted a selective advantage on swallows with shorter wingspans.

University of Tulsa Professor Charles Brown and his wife Mary Bomberger Brown, Program Coordinator of the Tern and Plover Conservation Partnership at the University of Nebraska at Lincoln, have spent nearly thirty years studying the American cliff swallows of western Nebraska. They frequent the environs of the University of Nebraska's Cedar Point Biological Station, located at the east end of Lake McConaughy, in the process having marked over 187,000 birds and collected hundreds of dead cliff swallows from roadside.

According to a 2005 U.S. Dept. of Agriculture, Forest Service report, an estimated 80 million birds are killed by colliding with motor vehicles each year in the United States. However, during the Brown's lengthy study of large colonies of cliff swallows, they found that the number of road-killed swallows has decreased significantly over that time.

The swallows being studied make their nests of mud, and roadways, bridges, overpasses, and road culverts are particularly favored sites. These man-made objects occur primarily near roads and motor vehicles, and as a result, so does most of the swallow activity.

American cliff swallows mainly eat insects, which they usually hunt on the wing. Well adapted to aerial feeding, they have a streamlined aspect and long pointed wings, and are capable of remarkable maneuverability. Swallows usually hunt at speeds of about 20-25 mph (30-40 km/h), but are capable of flying nearly twice as fast. When feeding, the chase is often fast, with a rapid flow of turns and banks. It seems rather clear that acrobatic flight absorbed in chasing food in the immediate vicinity of automobile traffic will indeed result in substantial collisions, in which the swallows are likely to come out second best.

So, it is reasonable to assume that a significant number of cliff swallows should be killed in traffic collisions. The odd thing that the Browns noticed is that, despite a considerable increase in both the swallow population in southwestern Nebraska and in motor traffic in the area, the number of swallows they found killed by vehicles each year has shrunk from about 20 in 1982 to 2-4 for each of the past two years.

Curious, the researchers compared various physical measurements of the birds found killed over the years (they had been preserved). They found that the wingspan of the road-killed birds was larger than that of the swallow population as a whole by about 7 percent. Also, while the wingspan of the road-killed birds increased by about the same amount each year, the wingspan of the general population decreased by that amount as well. As of 2012, the average wingspan of the sparrows is about 107 mm (4.2 in), while the average wingspan of the road-killed birds is some 4-5 mm longer.

To sum up, the data from the Brown's study shows that the swallows have become increasingly less likely to collide with cars, and the risk to all swallows is not identical – those with longer wings are at larger risk. Longer wings are harder to flap, resulting in slightly slower take-offs and maneuvers. This could explain the observations in terms of selective advantage favoring evolution toward shorter wingspans.

“The study shows evolution can work on very short time scales (30 years), and certain traits in birds can change,” Brown said.

The value of careful, thorough fieldwork is clear. This connection would have been so easy to miss, and instead the work is likely to prompt related studies on human interaction with other animals, and to provide a classic case study of evolution in the wild.

Sources: University of Tulsa, Current Biology (PDF)

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer. All articles by Brian Dodson

Cool! that's neat that they've studied them for 30 years, and now have enough data to show something like this that would otherwise go unnoticed.

It does make you think about how much selection processes are being effected by human interaction, directly or indirectly.



All real evolution in nature is within limits. The genes already exist for micro-evolution (variations within a biological kind such as varieties of dogs, cats, horses, cows, etc.), but not for macro-evolution (variations across biological kinds such as from sea sponge to human). The unthinking environment has no ability to design or program entirely new genes. Only variations of already existing genes and traits are possible. A dog will always be a dog no matter how many varieties come into being.

Evolutionists hope and assume that, over millions of years, random mutations (accidental changes) in the genetic code caused by radiation from the environment will produce entirely new genes for entirely new traits in species so that macro-evolution occurs. It’s much like hoping that, if given enough time, randomly changing the sequence of letters in a cook book will turn the book into a romance novel, or a book on astronomy!

Another problem for macro-evolution is the issue of survival of the fittest. How can a partially evolved species be fit for survival? A partially evolved trait or organ that is not complete and fully functioning from the start will be a liability to a species, not a survival asset. Plants and animals in the process of macro-evolution would be unfit for survival.

Imagine an evolving fish having part fins and part feet, with the fins evolving into feet. Where’s the survival advantage? It can't use either fins or feet efficiently. These fish exist only on automobile bumper stickers!

In fact, how could species have survived at all while their vital organs were supposedly evolving? Survival of the fittest (aka natural selection) may explain how species survive, due to minor variations and adaptations to the environment, but not how they originated. Natural selection merely “selects” from biological variations that are possible. It’s not a creative force.

Genetic and biological similarities between species are no proof of common ancestry. Such similarities are better and more logically explained due to a common Genetic Engineer or Designer (yes, God) who designed similar functions for similar purposes in various species. Genetic information, like other forms of information, cannot arise by chance, so it's more rational to believe that DNA or genetic similarities between species are due to intelligent design.

What about "Junk" DNA? The latest science shows that "Junk DNA" isn't junk after all! It's we who were ignorant of how useful these segments of DNA really are. Recent scientific research published in scientific journals such as Nature and RNA has revealed that the "non-coding" segments of DNA are essential in regulating gene expression (i.e. how, when, and where genes are expressed in the body).

All the fossils that have been used to support human evolution have ultimately been found to be either hoaxes, non-human, or human, but not human and non-human.

All species in the fossil record and living are complete, fully-formed, and fully functional. There's no macro-evolution in nature.

Visit my newest Internet sites, THE SCIENCE SUPPORTING CREATION and WAR AMONG EVOLUTIONISTS (2nd Edition)

Sincerely, Babu G. Ranganathan (B.A. Bible/Biology)


*I have given successful lectures (with question and answer period afterwards) defending creation before evolutionist science faculty and students at various colleges and universities. I've been privileged to be recognized in the 24th edition of Marquis "Who's Who in The East" for my writings on religion and science.

Babu Ranganathan


There is no such thing as a "partially evolved species", however individuals which show the transition between species are easy to find. An individual dog will always be a dog, but eons from now its descendants could be something entirely different that could never interbreed with a dog of today. That is how new species evolve. Nothing in the theory of evolution implies that a dog will have a litter of kittens. It does explain how dogs and cats have a common ancestor and how humans share a common ancestor millions of years ago with every living thing on earth.

You have a basic misunderstanding of evolution.

By the way, all you have to do to get into any Who's Who book is pay for it.

Suzanne Pratt

@Babu G. Ranganathan

"It’s much like hoping that, if given enough time, randomly changing the sequence of letters in a cook book will turn the book into a romance novel, or a book on astronomy! " Not at all. Evolution is gradual and there is selection pressure on each generation. It's more like "if a word is randomly formed and there is an external observer that does the selecting, in deep time there will emerge a coherent work of writing". The book is an analogy to the swallow and the external observer is analogous to the natural environment or, in this case, mostly with moving vehicles.


re; kwarks

The problem is that the intervening generations wont survive to reproduce. lets hope the long wing instructions do not bread themselves out of the swallow could be in real trouble. Imagine if the The peppered moth had lost the genes for the light color form during the coal blight.


Cars didn't produce Birds with shorter wings, and this isn't evolution. The physiological process which produced Birds with shorter wings, actual evolution, is entirely different to natural selection. Depending on their flight profile these birds with shorter wings could actually be evolving longer wings contrary to their suitability for life on the road side, but natural selection will keep it in check and they with the shortest will survive - however i imagine a low flying flight profile such as that along the road is producing shorter wings anyway, which makes sense as to why they'd be better at surviving cars (still nothing to do with the cars other than the cars are also at low level)

There's nothing random about evolution, or anything. "Random" is Human for "I don't know". An exact environment built an exact life form, and it continues to alter it

Suggesting that mutation occurs randomly works everywhere except real life, which limits it to the human imagination. I can imagine anything, i can imagine that babies are delivered by Stalks, the theory works right? The stalks just do deliver babies, there are babies and there are stalks, stalks can fly and babies can fit in blankets and therefor my theory works - except in real life. "Random" mutations might influence survival but it's not evolution, it's not the cause of a physiological change to the species

A "Random" mutation could either be:

1) Positive 2) Neutral 3) Negative

In which case the world would be full of animals with neutral mutations which make no sense whatsoever. However it's just not the case

Getting a uniform "random" mutation across an entire species is completely ridiculous, DNA is big, so to speak; the options for molecular change are difficult to imagine, after you remember that something has to cause it. The only "random" mutations are deformity and disease, and even they are simply caused by factors currently beyond our ability to predict in most cases

The old (and it is falling apart) "random mutation" theory completely ignores the sheer scale of it's own claim, and says "they just do evolve exactly the way we want them to and it is random because that theory is easiest to understand"

Kiel Stuart (@kielrhys)


You are so full of Babu that you should take a basic biology class. Genetic similarities are proof of common ancestry!!!!!

Greg Riemer

What an interesting discussion. Personally I think it's an act of faith to believe in Macro Evolution. The incredible amount of missing links should make any scientific mind suspicious. Irrespective, none of us were around a billion plus years ago to witness what life looked like, nor it's genesis. Whatever the causes, it's good to inquire why the changes are apparent. I'm skeptical if 112mm average wing diameter would equate to such a different performance to the 107mm wing diameter in terms of manuverability. It is only a 4.7% difference. Further, whatever losses are encountered would likely be offset by other advantages. Arguably a greater wingspan would provide better ability to rapidly reduce speed which creatures in nature also use to avoid collisions. A greater wingspan would also assist in taking advantages of thermals. This could mean the bird would expend less energy when hunting and so would need to consume less food for energy. This, in turn could mean less time on the wing, reducing chances of collision. IMHO there are too many factors to simply imply a greater wingspan is a disadvantage where vehicles are concerned. In my mind, the jury is (perhaps should be) still out.

Out of curiosity, does anyone know if there is any biological life on earth that is sustained by consuming purely inorganic matter?


That's not evolution. That's natural selection (google peppered moths & industrial revolution).

Ra'anan Elozory

"Out of curiosity, does anyone know if there is any biological life on earth that is sustained by consuming purely inorganic matter?"

Yes. Plants.

Rokdun Johnson

I wonder if the aerodynamic properties of the modern cars had any influence on the number of birds being killed.

Ilya Katsnelson

@Kiel Evolution happens in two major steps (simplified here): 1. Mutations in the DNA 2. Selection of those mutations that are best at surviving to breed and pass on the useful trait to the next generation

The first one is random, the second one isn't. That's why the statement that if you set a bomb off in a junkyard you won't get a 747 is a ridiculous analogy... that's totally random. If you've ever played Yahtzee, the dice throwing game (you can look it up) the chances of getting all 5 die showing up the same in 1 roll is very small. But if you keep those die that are the same and roll the rest, you'll get them all the same after just a few throws. The roll is random but the selection isn't. This is basic stuff that everyone should have learned in high School.


re; warren52nz

Nicely workable except that if in just one of the roles of the dice just one die bring up one of the four of the five wrong numbers the line dies. The die are loaded against winning as well.

Can you name a beneficial mutation?



Sickle cell anemia perhaps? One with such a condition has a survival advantage against malaria.

Bob Humbly

re; Bob Humbly

If you get the gene for Sickle Cell from one parent you are so sick that malaria doesn't find you a suitable host if you get the gene for Sickle Cell from both parents you die a horrible death. So while in malaria ravaged lands Sickle Cell does not breed itself out of the human genome it never spreads to more than about forty percent of the population. I can not call Sickle Cell a good mutation.


@Slowburn Gizmag isn't the place to have this dialogue. Look me up on YouTube if you want a lesson in high school Biology. In brief though, practically every mutation that has happened to every modern animal from its single celled ancestor to its present form is beneficial. It has to be otherwise it wouldn't survive the eons. As a specific example the mutation that happened in a certain bacterium that allowed it to digest Nylon (the new species is called Nylonase) is a rather prominent example.



This is NOT "evolution." This is called natural selection which is a very specific, observable process. BIG difference. "Evolution" is the term used by pop-science writers writing for a lay audience.

Then again, maybe that's why.

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