Correlation, Not Causation: Brood X Cicadas And Regional Bird Deaths NPR's Ari Shapiro speaks with Brian Evans from the Smithsonian Migratory Bird Center on the recent deaths of regional birds who ate Brood X cicadas.

Correlation, Not Causation: Brood X Cicadas And Regional Bird Deaths

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As trillions of cicadas emerged from their 17-year nap over the last month, something else started to happen. Birds that feasted on the insects died in large numbers. Now, it's not clear whether the cicadas were responsible for the bird deaths. What we can say is that the phenomenon tended to map onto the areas where the Brood X cicadas emerged. Brian Evans is a bird ecologist with the Smithsonian Migratory Bird Center, and he's been investigating this science mystery.


BRIAN EVANS: Thank you, Ari.

SHAPIRO: When did you start to notice something weird happening to birds in the region?

EVANS: I started just around the end of May. Some of our field teams had reported to us that they were seeing dead birds around the greater D.C. area and injured birds with a specific set of conditions. And that was blindness with crusty eyes and also these strange neurologic conditions that included shaking and what looked like general confusion among the birds.

SHAPIRO: And how big of a die-off are we talking about?

EVANS: We don't have great numbers locally on the size of this die-off. But we know in terms of the number of birds that have been brought to rehab centers living, we're at over 600 birds, so hundreds of birds at the least. But likely, that's a poor representation of the total number of birds impacted.

SHAPIRO: And what can you say about whether or not cicadas are responsible for this?

EVANS: Well, what we know is that the spatial extent, the area of the mortality event and the timing in the mortality event largely matches the emergence of the Brood X cicadas. But that's entirely correlation. We don't know directly whether it's caused, and we're trying to investigate the potential links that might be there.

SHAPIRO: And if it is tied to cicadas, is the best guess that it's linked to - I don't know, pesticides, weed killers, fungus - what?

EVANS: One of the guesses is that it's a fungus called massospora, and the fungus infects the cicadas. We don't know whether or not the fungus could actually negatively impact the birds right now, so we're exploring that as a potential cause. But pesticides are, of course, a potential cause. So these cicadas have lived underground right underneath us for 17 years. And in those 17 years, they could have been accumulating toxins like pesticides or heavy metals that then the birds could be exposed to at really high concentrations just because they've switched their diets over to cicadas.

SHAPIRO: Well, as the cicadas disappear, are the bird die-offs starting to diminish, too?

EVANS: It does appear that they're starting to go down. And that's an optimistic note. So we know that birds can really take once-every-17-year hit on their populations. But on the other hand, this could be the start of something else entirely.

SHAPIRO: Well, if you solve this mystery, will you let us know? We'll have you back on.

EVANS: Awesome.

SHAPIRO: Brian Evans is a bird ecologist with the Smithsonian Migratory Bird Center.

Thank you so much.

EVANS: Thank you, Ari.

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