This brown headed cowbird was one of many birds that feasted on cicadas—rather than caterpillars—when Brood X emerged in 2021.

In the eastern United States, billions of cicadas burst from the ground every 17 years to climb trees, mate, and lay eggs. The swollen insects provide as an unlimited banquet for birds, animals, and other predators for a few weeks. According to Kathy Williams, an insect biologist at San Diego State University, "it's a phenomenal explosion to an ecosystem."

According to study published today in Science, the effects of this gluttony spread across the ecosystem as satiated birds disregard their typical meal of caterpillars, who later gain weight by consuming the leaves of oak trees. "I thought this was an amazing study," says Jalene LaMontagne, a population ecologist at DePaul University who wasn't involved. Gene Kritsky, an entomologist at Mount St. Helens, says it "really adds to the big picture of cicadas."

Every year, a variety of cicada species can be heard chirping and buzzing throughout the summertime. Other species, known as periodical cicadas, live underground for 13 or 17 years before emerging in profusion. They emerge at various times and locations across the eastern United States in populations, or broods, that form a geographical patchwork. Periodical cicadas are produced in such large numbers by each brood that there are enough that survive to reproduce before being eaten by predators. Predators' populations may grow the following year as a result of the cicada feast, but before the cyclical cicadas reappear, they return to normal. 

Insects in oak forests have been the subject of extensive research by ecologists John Lill of George Washington University (GW) and Martha Weiss of Georgetown University. Lill and Weiss pondered the implications of the enormous number of cicadas for caterpillars as Brood X, the largest of the cicada broods, was still getting ready to emerge. Caterpillar populations can become out of control and cause damage to oak trees, but birds manage them. Birds might not have much appetite left for their typical diet of caterpillars, Lill and Weiss reasoned, if they fill up on cicadas.

They conducted a count of caterpillars on oak trees in two locations close to Washington, D.C., one year prior to the emergence of Brood X, to find out. By affixing clay caterpillar models, they and their colleagues also determined the appetites of birds.

The feast started in 2021 with the appearance of Brood X. 983 observations from birders in the Mid-Atlantic states were collected by Weiss and Lill. In total, 82 different bird species have been observed eating cicadas. At the time, entomologist Zoe Getman-Pickering was a postdoctoral researcher at the George Washington University. "We were really surprised at how many bird species were eating cicadas," she says. The 1.2-gram cicadas were wolfed down by trumpeter swans. Even tiny blue-gray gnatcatchers, weighing less than 7 grams, were able to pick at cicada parts like they were eating BBQ chicken.

But Kritsky isn't surprised. After all, Brood X cicadas are tasty and simple to catch. What would you do if you walked outside and discovered that Hershey's Kisses were flying everywhere?

With so much readily available food, birds didn't eat as many caterpillars. Fewer than 10% of the clay caterpillar models were pecked each week after the cicadas began to sing in May 2021. Those levels decreased to around 25% in August 2021, after the cicadas had finished mating and either died or were eaten. Real caterpillars were counted in the oak forests, and it was discovered that they had twice as many as in the two years after the cicada brood had returned underground. The eclipsed oak dagger (Acronicta increta), a common caterpillar, was also discovered to be

The enormously hungry caterpillars gorged on oak trees as they grew fat. The extent of the leaf damage was twice as great during the cicada glut, according to the researchers. Getman-Pickering claims, "The leaves looked like lace, they had so many holes of different shapes and sizes." Because the trees were too young to produce acorns, it is unclear whether such leaf damage hurt the trees' capacity to reproduce, but previous studies indicate that this level of herbivory could harm the forests. Oaks grow more slowly in years when cicada broods are active, according to research on tree rings.

When female cicadas lay their eggs inside twigs, the eggs themselves cause some damage. However, the cicadas' decomposing bodies also return nutrients to the soil, which might help plants. The tunnels and burrows dug by the bugs aid in soil aeration and water penetration.

Getman-Pickering hopes that the new study will serve as a reminder of how intricately linked ecosystems are, and that shocks to them can have far-reaching effects. The interference caused by Brood X "was brief, it was intense, and then it vanished."