As the first day of spring approaches a scientific mystery will soon return with a roar— the 2013 return of the east coast b rood of cicadas, or Brood II. Now a team of scientists hint they may have a solution as to why this brood and its fellows bizarrely emerge only after lulls more than a decade long—to control their surroundings in ways that may lead to crashes in numbers of predatory birds.
Periodical cicadas are the longest-lived insects known. After childhoods spent underground living off the juices of tree roots, broods of red-eyed adults surface in precise cycles— 13 years long in the southeastern U.S. and 17 years long in the northeastern part of the country. Fifteen broods are known to exist today on Earth, all native to North America. Brood II is set to emerge this spring in New York State, Connecticut, Maryland, North Carolina, New Jersey, Pennsylvania and Virginia with choruses of males bent on wooing mates with their din. It remains an enigma why these cicadas only emerge together in the adult stage every 13 or 17 years, as opposed to some other duration — other cicada species are not so synchronized.
"The 'periodical cicada' problem is one that's been kicking around for nearly 350 years at this point," says behavioral ecologist Walter Koenig at the Cornell Lab of Ornithology. "The first known mention of periodical cicadas was in Volume 1 of Philosophical Transactions, the first real scientific journal ever published, in 1665 . The fact that these insects still pose some of the most challenging problems there are in evolutionary biology is impressive."
A number of possible solutions to this mystery have been suggested over the years. For instance, researchers have speculated that the lengths of these strange cycles may reduce unwelcome interbreeding and competition for resources among broods — even if there was any geographic overlap of 13- and 17-year cycle broods, the lengths of the cycles would ensure that they only emerged simultaneously once every 221 years. Little such overlap exists, however, weakening this argument. Others have suggested these cycles may help periodical cicadas avoid predators or parasites that have shorter, even-numbered life cycles, but no strong evidence exists in support of these ideas either.
Now Koenig and his colleagues suggest the lengths of the cycles may somehow cause the numbers of cicadas’ predators—insectivorous birds—to drop. "Our hypothesis is, frankly, hard to believe, even by us, if for no other reason because we know of no mechanism to explain how cicada emergences could be driving a cycle in the birds that is so long—13 or 17 years,” Koenig says. “Because of this, I don't blame anyone for being skeptical. Unfortunately, it's not easy to study animals that only show up two or three times during one's scientific career."
The scientists analyzed 15 predatory bird species living within the geographic ranges of periodical cicadas that could potentially feed on the insects, including red headed woodpeckers, blue jays and gray catbirds. They focused on population data gathered over a 45-year period via the North American Breeding Bird Survey.
The researchers discovered the numbers of these birds dropped significantly during the years cicadas emerge. When it came to the 13-year broods, potential predator levels rose immediately after emergences followed by a crash in year four and a dip in year 10 as well as the emergence year. When it came to the 17-year broods, the levels of these birds rose slowly after emergences, peaking in year 12 and then declining afterward, reaching a nadir again in emergence years. Koenig and his colleague Andrew Liebhold detailed in the January issue of The American Naturalist.
It remains uncertain specifically how cicadas might orchestrate the behavior of other species over such long time periods. Koenig and his colleagues suggest the giant pulse of nutrients represented by a cicada emergence might influence tree growth and a host of other environmental factors, triggering a chain reaction that leads bird numbers to surge and then decline before the insects reemerge. "We are basically claiming that, at least indirectly, the cicadas are engineering bird populations and their environments," Koenig says. "Still, I don't claim we have a good mechanistic explanation for how that occurs."
Indeed, evolutionary biologist David Marshall at the University of Connecticut feels there is no conclusive evidence that birds’ numbers actually are declining. "The studies of the Koenig group have not yet sufficiently excluded a simpler hypothesis that can explain apparent bird population 'declines' during cicada emergence years—acoustic interference of loud cicada choruses with bird songs that are used by observers to locate birds," Marshall says. "In other words, there may be fewer bird observations in those years because the birds are more difficult to detect."
Koenig and Liebhold argue that bird levels in cicada ranges dropped during emergence years of broods even in areas where cicadas were not chorusing, suggesting their cacophony was not influencing reports of bird numbers. Marshall also notes, however, the current study treated sites as having cicadas present or not based on county-level distribution data. "Many papers have been published in the past decade and earlier showing the inadequacy of these maps. Periodical cicada populations are notoriously patchy, far more so than other North American cicada populations," Marshall says. "Adults can be abundant in one woodlot and completely absent one kilometer away."
"The cicadas are probably affecting the bird populations in some way—after all, many bird populations use the cicadas as food during emergence years—but I do not think that the patterns in this study prove that the cicadas are inducing delayed bird-population changes that occur 13 or 17 years after emergence," Marshall concludes.
As such, these periodic cicadas remain a cacophonous springtime mystery.