Unnatural Selection

Cliff swallow like a butterfly (photo by Chuck Tague)

Cars and trucks have changed the cliff swallow.

For 30 years Charles Brown and his wife Mary Bomberger Brown have studied cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska.  They've meticulously monitored, measured and banded the birds at their nests under bridges and overpasses and they've counted and measured the road killed birds.

Their attention to detail has paid off in an unexpected way.

Cliff swallows attach their mud nests to cliffs or bridges.  In Nebraska where there are few cliffs, the swallows use busy highway overpasses.   If the swallows aren't quick to fly up out of traffic they become road kill.

When the Browns began their study in 1982 they typically found 20 road killed cliff swallows per season, but since 2008 they've usually found less than five.  The traffic has remained the same while the swallows' population has more than doubled, yet the road kill numbers dropped dramatically.

What changed?  The swallows changed!

The Browns' data reveals that thirty years ago Nebraska's cliff swallows had longer wingspans.  Today's shorter wings allow the birds to maneuver more quickly and turn away from oncoming vehicles.  In fact, the few road killed birds they find today have longer wings than the rest of the population.

The shorter-winged birds survive to breed, the long-winged birds do not.  In only 30 years, traffic's unnatural selection has forced cliff swallows to evolve.

If traffic can do this to cliff swallows, I wonder what it's done to Pennsylvania's white-tailed deer.


Read more about this study in ScienceNOW.

(photo of a cliff swallow near the Rt. 528 bridge in Moraine State Park by Chuck Tague)

5 thoughts on “Unnatural Selection

  1. Alas, northern cardinals fly too low too. Twice in the past hour I’ve read of female cardinals hit by cars: 1 dead, 1 injured.

  2. Thanks for this post Kate….A Great Testament to what dedication by individuals can accomplish!
    About the deer:
    I’ve had two deer vehicle accidents, neither died… but got up and ran away…I experienced a large herd “out my window” :)… of deer that ran down a slope toward the county road I live on…with a large doe in the lead…She came up to a row of large pine that are off the road approx 50 ft and she stopped with all others stopping behind her…she surveyed ahead for so long that the yearlings began bucking about…she then slowly went through the pines into the yard beyond… but halfway to the road…where she stopped and focused across the road…(it’s a busy one traffic wise) She stayed put while a couple vehicles came and went down the road…when she made a decision to move forward, they all went as one…trotting until they reached the edge of the road. They appeared to not like their hooves to touch the road, as when they crossed some tried to jump OVER the road and others did a hip hop skip over it.
    But not a car came…Was she listening for cars? LOL…It sure looked like it!! Plus it was daytime and nothing was frightening them from behind the herd…

  3. My opinion on this.

    This is not evolution, this is adaptation. There is no change in the DNA structure here, thus no change in the species here. This is a particular species adapting to a new environment.

    Because of us humans, forced adaptation happens all to often in the animal world. Some species adapt easily, others not, thus they become endangered species.

    It is neat that a long careful study was done here. Thus, good science can come out of this.

    The beauty of science is it’s ability through accuracy like that here from this long study is to bring truth about something. We just need to be careful how we look at this good information and not quickly summarize something that may not be true.

    Again, simply my opinion,

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