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Cliff swallows adapt to urban living

NSF Award:

LTREB: Long term studies of social behavior in a colonial bird  (University of Tulsa)

LTREB: Demography and Disease Ecology of a Colonial Bird  (University of Tulsa)

LTREB: Long-term Studies of Demography and Social Behavior in a Colonial Bird  (University of Tulsa)

Fitness Consequences of Avian Coloniality  (University of Tulsa)

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Cliff swallows nest in colonies. As the name implies, they build their nests out of mud on the sides of cliffs or similarly shaped natural and manmade structures.  In the Midwestern U.S., some colonies establish themselves on the metal and concrete structures supporting highway underpasses. This location provides easy access for researchers and an excellent opportunity to investigate the dynamics of group behavior in a nonhuman species.

Researchers working with a unique multi-decade dataset have documented evolutionary change in a colony of cliff swallows (Petrochelidon pyrrhonota) under the selective pressure of automobile traffic. The team found that birds with shorter wings were better able to dodge highway traffic near the highway underpass nesting site and that this trait became widespread in the population over many years of observation.

This study, based on an unusually strong and extensive collection of long-term observational data, is a prime example of natural selection at work. It is likely to become a textbook example of how an environmental change (introduction of fast-moving automobiles) selects for specific adaptations in a population of organisms.

During their observations the researchers captured and measured live birds, and also collected and measured birds killed in the road at the site.  An informal observation that "road kill" birds in recent years seemed less common than during the early days of the study (even though the population had grown and traffic had not changed) prompted further investigation. Reexamination of the collected specimens revealed that birds with longer wings were more likely to become road kill, leaving birds with shorter wings more likely to survive and therefore pass their genes along to new generations.

Images (1 of )

  • a cliff swallow nest on a highway bridge
  • A mud nest under an interstate bridge
  • swallows killed by cars have longer wings than those that avoid cars
Cliff swallows nest on a highway bridge in southwestern Nebraska.
Charles R. Brown, University of Tulsa
Mud nests are clustered tightly together, close to speeding vehicles.
Charles R. Brown, University of Tulsa
Swallows killed by cars have longer wings than those that avoid cars.
Valerie A. O'Brien, Tulsa Community College

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