Periodical cicadas have such a strange life cycle that some have argued they can count, and have a particular affinity for prime numbers. That's because their broods emerge after lengthy periods of time; in North America, they appear en masse from underground every 13 or 17 years. Now, two researchers argue that the cicadas' cycles are timed to "engineer" the numbers of a mortal enemy—predatory birds. Contrary to what one might expect, these birds' populations drop significantly the year cicadas emerge in all their buzzing glory, the scientists report in the current issue of The American Naturalist.
"What we've found is so amazing, even I have a hard time believing it is true," says Walt Koenig, a behavioral ecologist at Cornell University and the lead author of the paper.
Scientists have puzzled over the cicadas' cycles since colonists first reported the insects' strange behaviors in the mid-1600s. All periodical species follow the same basic life cycle, living underground as nymphs for 13 or 17 years, and then emerging simultaneously in great numbers in the summer. Crawling up trees, they shed their skins, and become—for a few, brief weeks—glassy-winged adults that meet, mate, and lay eggs. After the eggs hatch, the nymphs descend into the netherworld again. But what natural forces drive this extraordinary cycle?
Knowing that researchers in the early 20th century had shown that avian predators can wipe out an entire population of cicadas that emerges out of sequence, Koenig decided to take a look at how bird populations might affect the insects' cycles.....
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