Periodical cicadas have such a strange life cycle that some have argued they can count, and have a particular affinity for prime numbers. That's because their broods emerge after lengthy periods of time; in North America, they appear en masse from underground every 13 or 17 years. Now, two researchers argue that the cicadas' cycles are timed to "engineer" the numbers of a mortal enemy—predatory birds. Contrary to what one might expect, these birds' populations drop significantly the year cicadas emerge in all their buzzing glory, the scientists report in the current issue of The American Naturalist.
"What we've found is so amazing, even I have a hard time believing it is true," says Walt Koenig, a behavioral ecologist at Cornell University and the lead author of the paper.
Scientists have puzzled over the cicadas' cycles since colonists first reported the insects' strange behaviors in the mid-1600s. All periodical species follow the same basic life cycle, living underground as nymphs for 13 or 17 years, and then emerging simultaneously in great numbers in the summer. Crawling up trees, they shed their skins, and become—for a few, brief weeks—glassy-winged adults that meet, mate, and lay eggs. After the eggs hatch, the nymphs descend into the netherworld again. But what natural forces drive this extraordinary cycle?
Knowing that researchers in the early 20th century had shown that avian predators can wipe out an entire population of cicadas that emerges out of sequence, Koenig decided to take a look at how bird populations might affect the insects' cycles.
Koenig and his co-author, Andrew Liebhold, an ecologist at the U.S. Department of Agriculture Forest Service in Morgantown, West Virginia, analyzed avian population data from the North American Breeding Bird Survey from 1966 to 2010. The scientists targeted 15 bird species, including yellow-billed cuckoos, red-headed woodpeckers, and house sparrows, which eat cicadas. Their analysis showed that these birds' populations reached their lowest point the year the insects emerged, which suggests that avian predators aren't exerting as much pressure on the cicadas as scientists thought. Indeed, 12 years after the cicadas' emergence, the birds went into decline, eventually reaching their lowest point in year 17—just when the cicadas were emerging again.
Birds dining on the 13-year-cycle cicadas followed a similar, if slightly more complicated path. Their numbers, too, reached their lowest point at the time of the emergence, and began to increase immediately after the cicadas appeared. "The cicadas are driving the birds' populations; they're setting the birds on a trajectory that leads to significantly lower populations at the time of the next emergence," Koenig says. The timing of the cicadas' cycles is all about manipulating their predators—and "may have nothing to do with these being prime numbers," he emphasizes.
So how do the cicadas manage to manipulate the birds over these long cycles? Koenig cites another study that reveals the enormous effect the biomass of cicadas has on the environment. "This is a huge resource pulse that enriches everything—soils, trees, and birds get a big nutrient hit. Now, it seems the aftermath of these emergences lasts even longer than we suspected—all the way to the next emergence."
"It's very intriguing, and offers a new explanation for why cicadas wait so long to emerge," says Richard Karban, an ecologist at the University of California, Davis, who is not involved with the study. "Avian predation has long been recognized to be a factor in these cycles, but Koenig and Liebhold convincingly suggest a way for it to work together with other ecological factors to produce the synchronized and long 13- and 17-year life cycles," adds Chris Simon, an evolutionary biologist at the University of Connecticut, Storrs, who has studied the cicadas' cycles but also was not involved with this work.
And yet mysteries remain. "We still don't have a good theory that explains how the cicadas' resource pulse causes the birds' populations to reach their low point exactly 13 and 17 years later," Koenig says. "We're still scratching our heads over that."