Signs of Summer 4: Interactions of Periodical Cicadas and Birds

Female cicada laying eggs. Photo by D. Sillman

(Click here to listen to an audio version of this blog!)

Science starts, as I have written before, with observation. You then have to filter through the sometimes bewildering array of events that you are observing and try to focus on those things that don’t quite make sense or on those things that seem to stick together. You can apply some creative thought (some imagination!) and begin to connect events with lines of cause or influence. You can then make explanatory models (or “hypotheses”) about/with these edited observations. These models are very satisfying and often incredibly elegant and beautiful! The critical thing about science, though, is the intellectual process doesn’t stop here. In science, we then challenge our precious hypotheses via experimentation and try to find their weak points. We try to show that our insight into Truth was, in fact, false!

Science, when it is working properly, is a very bruising ego-trip!

Which gets us, interestingly, to periodical cicadas and some of the bird species that we might expect to consume them!

A simple prediction about a periodical cicada outbreak year (year #13 or #17 in their life cycle depending on which cicada species or brood is involved) is that birds that can eat those cicadas should become quite abundant when their food supply is so dramatically increased! This predictive hypothesis seems so logical that it feels foolish to even test it!

American crow. Photo by D. Sillman

So, as the local periodical cicada outbreak raged on here in Apollo this spring, I was very surprised to observe that the numbers of cardinals, blue jays, crows, and grackles (all species that should be eating cicadas) in and around my yard, field and woodlot had plummeted!

Over the past 15 years I have regularly had 20 to 30 northern cardinals in the area immediately  around my house. They nest in my cedar and spruce and maple trees and, usually, each mating pair will have three clutches in a spring/summer. They wean their fledglings at my sunflower seed feeders over a multiple days long ritual of wing fluttering and begging and parental attention and then abandonment. This year, though, I have seen only 1 or 2 pair of cardinals and just saw my first fledgling a few days ago! Also the crows and blue jays that have regularly greeted me in the morning to receive their daily allotment of peanuts in the shell have been quiet this summer. Many peanuts dumped in my front yard in the morning sit untouched by birds (but, eventually, they get gathered up by squirrels!). And, the grackles just are nowhere to be seen!

This has been an odd spring/summer for birds in Apollo!

Cicada damage on a black locust branch. Photo by D. Sillman

Walter Koenig, a scientist at The Cornell Laboratory of Ornithology, has made a number of observations on bird species during cicada-emergence years. Koenig looked at the North American Breeding Bird Survey (a comprehensive survey of North American bird species that has been conducted every year since 1966) to see if he could find any evidence of possible correlations of the periodical cicada outbreaks with the fluctuating population densities of 24 species of potential periodical cicada predators. His analysis indicated that fifteen of these bird species had statistically significant population changes related to the cicada life cycle.

Cuckoos (Coccyzus spp.) were found in high numbers only during a periodical cicada outbreak year.  Red‐bellied woodpeckers (Melanerpes carolinus), blue jays (Cyanocitta cristata), common grackles (Quiscalus quiscula), and brown‐headed cowbirds (Molothrus ater), on the other hand, were found in higher densities for 1 to 3 years after a cicada emergence). These data fit the “expected” (or “logical”) model of stimulation of predator species when prey becomes increasingly abundant. Other potential cicada predators, though, responded very differently to the periodical cicada feast.  Red‐headed woodpeckers (Melanerpes erythrocephalus), American crows (Corvus brachyrynchos), tufted titmice (Baeolophus bicolor),

Blue jay. Photo by D. Sillman

Tufted titmouse. Photo by D. Daniels, Wikimedia Commons

gray catbirds (Dumetella carolinensis), and brown thrashers (Toxostoma rufum) were found in very low numbers during a cicada outbreak year. Their population densities declined when the cicadas were available but then increased back to “normal” the year after the outbreak and then for several years remained at their “normal” densities. Wood thrushes (Hylocichla mustelina) and northern mockingbirds (Mimus polyglottos) both had significantly reduced populations 1 to 2 years before the cicada outbreak but then returned to “normal” numbers during the outbreak and in the years after. Northern cardinals (Cardinalis cardinalis), and house sparrows (Passer domesticus) had significantly increased populations 1 to 2 years before the cicada outbreak followed by a return to “normal” population densities during the outbreak and in the subsequent years of the cicada life cycle.

Further, even though all of these bird species have inherently and distinctively fluctuating population densities , most of the potential cicada predators reached the lowest population numbers of their cycles the very year that the periodical cicadas were expected to emerge. These emerging cicadas, then, were preyed upon by a much smaller predator community than average! A very big advantage for the cicadas!

The classical description for the evolutionary selection of periodical cicada mass emergence and their very long subterranean nymph stage existence is predator saturation, and, subsequently, increased survival of reproducing adults. There seems to be another dimension to this strategy, though, that involves coordinating the long-term density fluctuation patterns of the cicadas’ main predators to the duration of the cicadas’ life cycle!

Koenig also looked at the data collected in the Audubon Society’s “Christmas Bird Count,” and found, six months before any adult cicadas might be emerging, reduced numbers of crows, blue jays and several other potential cicada-eating species. The decline, then, in these birds was something that was occurring long before there were any actual adult cicadas in the ecosystem!

Cicada exoskeletons after emergence. Photo by D. Sillman

A  question that has been historically asked is, “why do the cicadas have cycles of 13 or 17 years?” Why are these two prime numbers uniquely used in these spectacularly extended life cycles? Cicadas that abnormally emerge at years other than 13 or 17 years are almost all consumed by predators, and these populations tend to go extinct. Further, computer simulations in which cicada emergence was programmed to occur in a variety of even numbered years also resulted in the local extinction of the hypothetical broods.

Why are years 13 and 17 so important? The more mathematical among us point to the fact that they are both prime numbers (but, the purely mathematical explanation seems to end there). The more mystical among us might contend that 13 and 17 have innate, hidden powers and meaning on other planes of reality. The most logical scientific explanation that I have heard was from Walter Koenig himself in an interview with Discover magazine in June 2013: “ … something really cool (is) going on that we just don’t know about yet.”

Cicada damage on black locust trees. Photo by D. Sillman

Possibly, the infusion of such a bounty of food energy in the cicada mass emergence acts as a controlling force on the fluctuating populations of the potential cicada predators that leads inevitably to the predators’ population density nadirs just when the periodical cicadas are programmed to emerge! Possibly the maturing cicada nymphs underground are altering their ecosystems’ productivity as they get larger and feed more aggressively on their sustaining tree roots which then causes potential predator populations to decrease.

As Koenig and Andrew Liebhold wrote in their July 2005 paper in Ecology (Ecological Society of America):

“These results suggest that the pulses of resources available at 13‐ or 17‐year intervals when periodical cicadas emerge have significant demographic effects on key avian predators, mostly during or immediately after emergence, but in some cases apparently years following emergence events.”

So, as scientists what is next? We have an elegant and very intriguing hypothesis: the mass emergence of cicadas is controlling their potential predator densities 13 or 17 years into the future! Natural selection has zeroed in on 13 and 17 years not because they are prime numbers or have mystical meanings, but because they match the period frequencies of the birds’ population fluctuations! Hypotheses, though, are meant to be attacked and taken apart. Stay tuned for more updates (sometime in the next 17 years)!




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