Cliff Swallows' Nesting Loyalty Can Be Hazardous

The blood-sucking bedbugs that invaded New York city in the fall have relatives, Oeciacus vicarius, which are helping to diminish your chances of spotting cliff swallows this spring.

The insects and mites infest the birds' unique gourd-like shaped nests, able to survive several years in unoccupied nests as they wait for their bird prey. Cliff swallows expert Charles Brown, of the University of Tulsa, counts these parasites among the most likely causes of the steady decline of swallow nests, and sightings, since the early 1990s in San Juan Capistrano. 

He educated an audience about the parasites, and other detriments, during a lecture on St. Joseph's Day—a celebration marking the onset of spring and the return of the swallows from their winter homes in Argentina—at Mission San Juan Capistrano, once a destination for cliff swallows to build colonies.

Parasites have affected bird populations severely, Brown said while displaying images of a sickened hatchling from an unfumigated nest site next to a fumigated one. In colonies where nests are not fumigated—a method used to kill the bugs—baby swallows have been seen jumping out of their nests, only soon to be picked up by scavengers.

Indeed, nestlings seem to be most at risk. UC Davis researchers found that cliff swallow colonies will not be reoccupied after one or two years because of heavy infestations. The swallows have even been found to "desert their nests en masse, leaving their young to starve, when swallow bug populations become too great."

In one of his own books, Coloniality in the Cliff Swallow, Brown called the parasites "undoubtedly one of the most important costs of living in colonies for cliff swallows." When parasites infect a nest, it reduces an adult swallows' chances of survival by 33 percent, and the bugs cost nestlings, on average, up to 3.4 grams in body mass, and reduce chances of survival by up to 50 percent.

Brown, who has been studying the species for nearly 30 years, discussed why swallow colonies in the city have declined within the last two decades, return around the same time each year and the risks the birds face in terms of parasites and other issues here and in Nebraska, where Brown does a majority of his studies.

Another reason of the declining numbers of migrating swallows is a lack of food source and a treacherous journey.

“If [swallows] return too early, it’s risky because you can get killed by the cold weather,” Brown said referring to the return of the birds in March.  “We had a cold weather event in 1996 in our study area, and we had a period of six days of cold and rainy weather that killed about half of the population, so you can’t come back too early.”

But "a better explanation might be the increased forest cover in this particular area,” Brown said. “If you look at photos of this area in was open—there were no trees around."

"One hypothesis is that the increased numbers of invasive tress have basically changed this area into a more forested ecosystem as opposed to an open one, and that has led to the decline of these birds here in terms of their habitat requirements.”