What the Buzz? Cicadas and Bird Populations
If you are in the Northeastern United States, you might be tired of hearing about the emergence of 17 year periodic cicadas. Cicadas are a type of arthropod in the order Hemiptera (“true bugs”), alongside aphids and shield bugs. The cicadas that have capture scientists’ attention this summer are the 17 year periodic brood. These insects are appearing en masse across the northeastern United States, and you’d think the birds might be thrilled to have such a wonderful outpouring of food resources as they raise their young this summer. But there’s a mystery afoot! Insectivorous bird populations usually decline before this phenomenon occurs, and increase after the cicadas are long gone. As of now, three hypotheses exist to explain this bizarre occurrence. Two researchers, one affiliated with the Cornell Lab, are using this emergence as a way to investigate some hypotheses, and they argue that the cicadas’ cycles are timed to “engineer” the numbers of their mortal enemy—predatory birds.
The Detectability Hypothesis:
Imagine a baby crying in a restaurant. Now, imagine trying to have a conversation over that crying baby. In nature, the cicadas are like the crying baby, while the birds are the individuals trying to have a conversation. Many species of birds feed in groups, and their ability to communicate and locate individual cicadas may be severely impaired by 120 decibels of buzzing; however, this does not explain why the bird populations decline before they emerge.
The Repel Hypothesis:
As mentioned before, a single male cicada is capable of producing sounds at 120 decibels, loud enough to approach the pain threshold of the human ear. The repel hypothesis argues that the sound is loud enough that the birds will actively avoid areas populated by cicadas.
A study conducted by Smith et al in 1985 found that Red-winged Blackbirds were more likely to successfully prey on cicadas that were silent and motionless, rather than loud and active, supporting this hypothesis. But this still does not explain why bird populations decline even where cicadas are not present.
Walter Koenig, researcher at the Cornell Lab of Ornithology, supports the true decline hypothesis, which argues that periodical cicadas are somehow modulating bird populations through mechanisms not known yet. It has been suggested, according to Koenig’s recently published paper, that the emergence of cicadas can affect bird populations for several years thereafter, possibly due to the destruction cicadas cause to trees through drinking their sap and laying their eggs in their branches. Another influencing factor involves the timing of the cicada’s life cycle. Their periodical emergences occur in prime numbers, which reduces the odds that predators (which usually have life span cycles of 2-5 years) will experience a population surge during cicada emergence years.
It’s a fascinating science question and a reminder that even life-long scientists don’t fully understand all the intricacies and mysteries of nature! Read more about this work at the Cornell Lab’s Round Robin Blog and at this Discover Magazine post, and encourage your students to ask and answer their own questions (about cicadas or otherwise) using our Investigating Evidence curriculum (available for a free download).