|Nesting Under A Bridge|
Cliff swallows, Petrochelidon pyrrhonota, have adapted to humans by adopting our structures, building their nests on building ledges, barns, houses and increasingly on bridges and overpasses. These have surfaces off the ground with a ledge for their nest foundation. They feed mainly on flying insects and need nearby open fields for their aerial insect hunting. Swallows have long pointed wings to sustain them swooping gliding flight as they nail their prey in midair.
These swallows are learning to cope with one of the hazards of modern living - automobile accidents. A dedicated 30 year study of cliff swallows in Nebraska by Charles and Mary Brown showed that while there are more nests on the overpasses, the mortality rate from being hit by cars is dropping. And it may be more than learning about traffic safety.
When they studied the birds, they discovered that the roadkilled swallows had longer wings than their surviving neighbors. In fact, they found that the cliff swallows of Nebraska have had a gradual decrease in the length of their wings over the last 30 years. They postulate that this population may be evolving to meet the challenge of traffic.
Shorter wingspans mean that they are more maneuverable and able to take off vertically to escape the traffic. Increased survival may provide more reproductive success for the shorter winged swallows, therefore passing along their individual genetic trait.
Whether this is beneficial in the long run remains to be seen. When autumn comes, they still have to gather up in flocks to make the long migration to Central and South America, a trip that would probably be easier if they had the longer wings. We will check back on this story in another 30 years to find the answer.
I ran this story by Charley Burwick, my resident bird guru and got this response:
"There is a really cool Cliff Swallow colony under the Hwy 65 south bridge over James River near the National Cemetery. I study it each year to see if a Cave Swallow might show up in the state. It is supposed to happen one of these days as the Cave Swallow reports keep moving closer and closer to Missouri. The Cave Swallows will mix in with the Cliff Swallows."
Note: Cave swallows are medium-sized, squarish tailed swallows in Mexico with some breeding colonies in New Mexico and Texas. They belong to the same genus as the more familiar and widespread Cliff Swallow of North America. Their coloration differs subtly from cliff swallows. With warmer climates moving gradually northward, followed by armadillo and other southern species taking up residence, can the cave swallows be far behind?