When big broods of cicadas emerge every 13 or 17 years, birds are treated to a smorgasbord of fresh food – and this sudden glut has cascading effects on other animals and plants in the ecosystem.
John Lill at The George Washington University in Washington DC and his colleagues first noticed the huge ecological impacts of cicadas in 2004. They immediately began planning to study the next of these once-in-a-generation events, when that huge brood, known as Brood X, would emerge again across the eastern US in 2021. “We had 17 years to think about it,” says Lill.
The researchers suspected that the sudden appearance of cicadas would cause birds to opportunistically change their diet to focus on the new food source, leaving their usual prey, like caterpillars, temporarily uneaten. They set out dummy caterpillars made of clay and recorded the telltale marks left by the beaks of confused birds as they attempted to eat them.
The team found that, in years with no cicadas, about one quarter of the dummies were attacked each week, but during the short few weeks of cicada season, fewer than 10 per cent showed signs of bird strikes.
Lill and his colleagues also enlisted local birders to observe birds feeding on cicadas, finding that more than 80 different species were taking part in the all-you-can-eat cicada buffet – even ones that don’t normally eat insects.
“They saw owls, swans, herons and even small songbirds whose beak we thought would be too small to eat a cicada,” says Lill. “Some didn’t recognise the cicadas as food at first, but they eventually figured it out.”
That temporary relief from predation had a huge impact on caterpillar populations and the forest at large. The team observed more than twice as many caterpillars during cicada season, and those caterpillars caused twice as much damage to trees and leaves as usual. “In a normal year, birds regulate insect herbivore damage, but that gets disrupted in cicada years,” says Lill.
These impacts are short-lived and the trees soon recover, but other studies have reported more enduring effects. The populations of some birds are higher in the year after an emergence and the cicadas can influence the timing of oak tree “masting” events, when the trees produce unusually large numbers of acorns.
The research gives a preview of what a world with fewer birds might be like, says Lill, as their populations dwindle due to climate change and other human interference. “Birds are important for regulating insects in forestry and agriculture,” he says. “Without them, there will be more damage to forests and food crops.”
David Beresford at Trent University in Peterborough, Canada, says the study highlights the importance of looking for the unexpected outcomes that can result from changes to species in an ecosystem. “We’re not always going to see the effects where we expect them to show up,” he says. “There can be ripple effects across the whole system.”