Cliff Swallow Adaptation

One of our Christmas books from Son Matt was Darwin Comes to Town by Menno Schilthuizen. This excellent book discusses how urbanization is accelerating and changing evolution.

In the book are many examples of HIREC, Human-Induced Rapid Evolutionary Change. The classic textbook example of HIREC is the peppered moth (Biston betularia) in Manchester, England during the industrial revolution. The peppered moth changed from a sprinkling of black marks on a white background color pattern to anthracite-black when the soot from the coal-powered  textile industry turned everything in the environment black, including the light-colored birch trees on which the peppered moth rests during the day. After regulations were put in place to reduce industrial pollution, the blackened trees of Manchester returned to their original coloration and the peppered moths returned to their white/black form.

Another example of this rapid evolutionary change was demonstrated by Charles Brown and Mary Bomberger-Brown in a 2013 Current Biology (Volume 23) article. This pair spent 30 years (1982 to 2012) studying cliff swallows (Petrochelidon pyrrhonota). Cliff swallows make gourd-shaped mud nests on vertical walls under horizontal overhangs, historically rock overhangs and cliffs. With the appearance of highway overpasses, bridges and culverts, cliff swallows began to colonize these new structures. These two biologists collected dead cliff swallows from roadsides in Nebraska and also used mist nets to collect living cliff swallows. (I will not go into the details, but they used strict scientific methods and controlled variables in their study.)

After 30 years of counting and measuring the wing length of dead and living cliff swallows several trends appeared:

*Although the wing length of dead and living birds was approximately the same when the birds first began using roadside structures for nesting, over time the wing length of the living birds grew shorter than those of the dead birds.

*The wing length of road kill birds was longer than the population at large and as the observations continued the divergence increased.

*Over time the number of road-kill swallows decreased from the time they first began to use highway nesting sites, even though the overall populations of “highway cliff swallows” (number of nests) increased significantly.

The conclusion of their study was that shorter wing length is an adaptation that helps cliff swallows evade vehicles. Longer wings have lower wing loading and do not allow as vertical a take-off or as much maneuverability, Short-winged cliff swallows sitting on the road (as they often do) can fly upward more vertically and thus avoid vehicles better than long-winged birds. Cliff swallows with short wings flying about the road can also pivot and maneuver away from hazards better than long-winged swallows.

The authors of this study admit that there may be other explanations, other than wing length, for the decrease in cliff swallow mortality (the birds “learn” to avoid collisions, changes in their insect prey, weather), but they do not see evidence for other influences.

It appears as though over the brief period of 30 years cliff swallows rapidly evolved shorter wings in response to their human influenced environment.

The gourd-shaped cliff swallow nests were photographed at Hydroelectric Plant #1 along Hat Creek. The cliff swallow was carrying mud from the shore of Baum Lake to its nearby nest under construction. Both sites are in Shasta County CA.

More information on cliff swallows may be found in several of my previous posts including “Cliff Swallow” on 05-15-13.

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2 Responses to Cliff Swallow Adaptation

  1. usermattw says:

    Glad you enjoyed the book! 🙂

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