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.