It isn’t Spring until the cliff swallows (Petrochelidon pyrrhonota) return to Cabrillo, a community college located in the hills along the Central Coast. Like their famous counterparts, the legendary swallows of Capistrano, these winged harbingers of warmer weather regularly return to campus every March. Although their migration schedule stays the same, recent research shows some swallows that prefer to nest in man made structures—instead of the cliff-side dwellings they are named for—may be changing their wingspans to increase their survival rates. 

 One of the people who await the campus arrival of the iridescent blue-backed birds is Michelle Merrill, an adjunct professor of anthropology. The “scouts” start showing up in early March to start building nests, says Merrill. They’ll stay through August, until the young are ready to make the long trek back to South America.

On the Cabrillo campus, Merrill points out the gourd-shaped nests found in clusters tucked under the second-story eaves of the library, bicycle co-op, and cafeteria buildings. The area is unofficially called the “Plaza de las Golondrinas,” says Merrill, the Spanish translation for plaza of the swallows.

Each nest is made of mud, one almond-sized daub at a time, then lined with grass. The male and female of nesting pairs both contribute to building the labor-intensive structure, she says. At Cabrillo, the birds ferry their beaks full of mud from the well-watered football field nearby. The swallows may take hundreds of trips to complete a new nest. With that kind of work, it’s no wonder the birds reuse nests built in previous years—as long as their former homes haven’t hosted too many parasites in the meantime.

“At a distance, I can tell the swallows apart from other birds by their flight pattern,” says Merrill. “They zip and dart, changing direction really fast.” Those aerial maneuvers allow the birds to catch their insect meals in mid-flight. 

But, when these birds nest in bridges and highway overpasses, their flying skills may be no match for the vehicle traffic that shares the same habitat. Swallows that nest in these risky places often get recorded as roadkill. 

The nests are so commonplace in overhead roadway structures that CalTrans studied ways to prevent cliff swallows from building mud-dwellings there. The birds’ presence prevents repair work during nesting season. Even though cliff swallows aren’t an endangered species, their migration route across two continents makes them eligible for protection under the Migratory Bird Treaty Act; it’s illegal to remove their nests from February 15 to September 1 without a permit from the U.S. Fish and Wildlife Service. 

Although CalTrans tried construction with special slippery siding and broadcasting distressed bird calls, the swallows didn’t change their nesting spots. But, living in proximity to the cars may have changed their genes, say researchers Charles R. Brownand Mary Bomberger Brown in a 18 March paper published in Current Biology.

After following the cliff swallows for three decades and consistently logging data on roadkills in a specific area of southwestern Nebraska, the two scientists noticed a steady decline in the number of swallows killed by cars—despite an increase in the population overall. Moreover, the Browns discovered that among the dead birds they collected, those with longer wingspans were killed more often than their shorter-winged kin. 

The researchers noted factors other than short-wing selection could be at play, such as learned avoidance of cars, selection against risk-taking birds, or changes in food sources. But with thirty years of data, they theorized that survival of swallows with shorter wing spans could be creating a population shift. The short-winged swallows could be better able to accelerate straight upward or “pivot away” if threatened by a moving car; therefore, more short-winged birds would survive to reproduce and pass along their genes. 

If the cliff swallows are changing as a result of their urban nesting preferences, they’ll join other avians—such as the high-rise nesting peregrine falcons—adapting to man made environments and increasing their survival. That’s a Spring herald worth waiting for.