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Since 2004, billions of clear-winged insects have been growing just beneath the surface of the eastern United States. Over the past year, their eyes turned red, one last change before the bugs are ready for their debut. Finally, this spring, on a warm night in May when the soil reaches 64 degrees, the bugs will use thick forelegs to dig a tunnel to the surface. They’ll wait until the timing is just right. In the middle of the night, each nymph will emerge from its hole and climb a nearby tree, hang upside down, and perform a sit-up. The force will split its exoskeleton down its back, wide enough to release a broad insect with a soft, white body. Through the night, crumpled wings will expand and straighten. Their ghostly exteriors will darken to black with orange spots.

People will awaken that next day to meet Brood X. With the noise of their courting calls, they'll be hard to ignore. 

Cicadas crawl about forests all over the world, and most have annual life cycles. But periodical cicadas, which don’t exist  on Earth but the eastern United States, spend far more time underground before emerging in broods to mate either every 13 years or every 17 years. A different cicada brood emerges most years, but these groups vary greatly in size and location. This year will be special: The so-called Brood X is among the largest and densest. The insects will appear by the billions in that cross parts of Pennsylvania, Delaware, Washington, D.C., Illinois, Ohio, Indiana, Georgia, and Tennessee. “There’s no end to how bizarre and interesting these cicadas are,” says Walt Koenig, a research zoologist emeritus at the University of California, Berkeley, who, even in retirement, is still working to unravel their mysteries.   

Scientists are still not certain why these insects evolved these unusually long, coordinated life cycles. One relatively straightforward explanation is the idea of safety in numbers, says University of California-Davis entomologist Richard Karban. Birds usually help keep bug populations in check, and the cicada influx will be a bonanza for a number of medium- or larger-sized birds that are big enough to dine on the two-inch insects. But because millions of cicadas emerge on a single acre of forest during big brood years, even the keenest birds hardly put a dent in the population. “The vast majority escape getting eaten and live to lay the next brood,” says Karban. 

But some scientists believe the story of how birds and these insects interact may be more nuanced or complicated than that. 

In Costa Rican rainforests, researchers understand that tropical birds usually  where cicada noise is the loudest, which can drown out their own calls. , Koenig and a colleague at the U.S. Forest Service thought this might also be the case with the eastern United States' periodical cicadas. Field researchers had noticed that certain bird populations appeared to be lower in the years that periodical cicada broods emerged in their territories. Maybe the cicadas were the cause, but it was also possible the birds were being undercounted because scientists couldn’t hear them as easily on their surveys, which often rely on identifying birds through their sounds. So the pair went to test this. To do so, they compiled 37 years of data from the , focusing on 24 insect-eating species that live in the periodical cicada range.

It wasn’t a sonic illusion—the numbers of 15 species appeared to ebb and flow in step with periodical cicada broods in their territory. Red-headed Woodpecker, American Crow, Tufted Titmouse, Gray Catbird, and Brown Thrasher populations were smaller in cicada years compared to years without cicadas. Their numbers also appeared to spike the following year and then stabilize. The study also showed that the number of Red-bellied Woodpeckers, Blue Jays, Common Grackles, and Brown-headed Cowbirds in an area increased sharply one to three years following a feast of cicadas, then declined after that, suggesting that populations swelled to meet the abundance of food, and then dwindled when not as many of their species survived during cicada off years. 

Some researchers, Koenig says, took these findings to mean that cicada broods may time their entrance with years when bird populations would be at their lowest. Others believed that the data hinted that periodical cicadas were engineering those populations themselves. “Even I think that’s a crazy hypothesis, but it’s not any crazier than what anyone else has at this point, and there is at least some data to back it up,” Koenig says.  

All of this makes this year's Brood X emergence a scientific opportunity. For example, Zoe Getman-Pickering, a postdoctoral research entomologist at George Washington University, is among a new generation of scientists who will experience the phenomenon of Brood X for the first time this year. She’s part of a group of researchers who hope to better understand the ripple-down effects that occur when crows, robins, and other medium-sized birds temporarily switch from eating caterpillars and other insects to cicada meat. She expects the diet shift will have a broader effect on forest ecosystems, including on smaller birds.  

“We want to know: Will smaller birds eat enough caterpillars to keep these populations in balance? Or will the caterpillar populations get out of control and wreak havoc on the forest if medium-sized birds aren’t eating them?” she says. “Birds play a really important role. How many birds there are, and what they’re eating, affects the entire ecosystem.”