Swarms of tasty cicadas don’t help the birds — what gives?
Every thirteen years they come. After over a decade underground, they build burrows to the earth’s surface and emerge in synchrony, clawing and crawling up through the soil, rip their skins down the back and are reborn as adults. And after a month, they will be dead, whether consumed by the animals awaiting their arrival or as a part of their lifecycle, with the females having laid eggs in the soil to develop for another thirteen years.
Some cicadas emerge every single year — annual or “dog days” cicadas — but two broods are on much lengthier cycles. This year, 2011, Brood XIX cicadas have already begun emerging throughout the southern United States. In 2004, I was lucky enough to experience the emergence of Brood X and won’t have the honor again for yet another decade, after a total of 17 years in 2021.
Their peak in 2004 coincided with Princeton University’s alumni weekend — which I attended as a bored high schooler — and what poor timing for that event! The endless drone of the insects forced us to yell just to make conversation; the air was so dense with their swarms that they would fly into unsuspecting Princeton grads with a size and velocity that was actually painful; sidewalks and windshields were splotched with the pale green stains of the squished deceased.
Besides providing a weekend of entertainment for high school hooligans witnessing the torture of exasperated ivy-league graduates, the emergence of millions upon millions of cicadas for just a single month provides an ephemeral pulse of resources. Once dead, their decaying bodies add nutrients to the soil (1) and streams (2), increasing soil microbes (3) and detritivorous insects (4), while their predation on roots as nymphs and plants, young ones in particular, as adults can decrease tree (5, 6) and plant (7) growth in the cicadas’ emergence year.
This deluge of huge, nutritious bugs should be a boon for their predators as an undepletable food source. While one study (8) found no change in a population of white-footed mice after Brood X emerged, the same study saw the number of short-tailed shrews increase four-fold — now that’s a lot of shrews!
So you’d probably expect a similar reaction from insect-eating birds — that some would have no change, but some would benefit greatly from the cicada surge, especially since their migratory nature would allow them to gather by the cicadas during emergence years. But when ornithologist Walter Koenig and entomologist Andrew Liebhold compared populations of 24 bird populations over the course of 37 years from the Breeding Bird Survey data set with periodic cicada pulses (9), they found only two species that showed up just for the cicadas — both North American cuckoo species. Out of the other 22 species, only 6 populations increased; the remaining 16 declined, and 5 of them with statistical significance.
They had three hypotheses to explain why so many bird populations were decreasing, according to a large observation-based bird survey, even when the birds were practically drowning in ample food resources. The overwhelmingly loud noise of cicada calling could drown out bird calls for the birdwatchers, creating an observer bias artificially lowering the reported number of birds in the area — the detectability hypothesis. Alternatively, all the racket could keep the birds from hearing one another, disrupting their communication and driving them to quieter areas with fewer cicadas — the repel hypothesis. Or the birds populations could be declining for another reason, affecting them beyond the ranges of the cicadas — the true decline hypothesis.
Koenig and Liebhold just published a reanalysis (10) in March 2011 in Ecology to test their ideas about why these bird populations are dropping. Again, they used the Breeding Bird Survey data for the 12 species showing the greatest decline from their previous paper, but also compared populations to the counts from the previous winter (Christmas Bird Counts) and incorporated notes about cicada prevalence from the counts where they could.
Their results supported the true decline hypothesis — that the birds’ population declines are not related to the emergences at all. They suggest that the drop in population numbers could be an indirect effect of the cicada emergences, however. The voracious plant consumption of the cicadas could be negatively affecting other insect prey sources or otherwise adversely impacting the immediate habitat. Additionally, while North American cuckoos are not typically nest parasites, this behavior — laying their eggs in the nests of other birds — has been observed when cuckoos are under intense competition with one another, as in this situation.
Some scientists (11) suggest that cicadas emerge on such odd timescales — 13 and 17 years, I mean, c’mon! — specifically to mess with their predators. If they emerged too frequently, their tactic of completely overwhelming the area with their presence, basically guaranteeing that many of them will successfully reproduce no matter how many housecats and cuckoos are fed — predator satiation — wouldn’t work. If they emerged more frequently, the gains of their predators from the cicadas’ previous emergence may still be lingering, effectively increasing their predators’ numbers each year until the cicadas themselves were overrun. (A hard scenario to imagine, I know — that would take even more shrews!)
But could the cicadas have evolved to take advantage of such a long-term concept? That they would have to effectively “wait” for their predators to be on the decline before they emerge again? Some biologists have even hypothesized that the latency periods of 13 and 17 years are significant because they are prime numbers! Take it away, Dr. Nicolas Lehmann-Ziebart:
[A]ssume that cicada predators consist of species having cyclic or ‘‘quasi-cyclic’’ dynamics with either two- or three-year periods. This leads to high predator abundances, and high predation rates, in years divisible by either two or three. Because primes are the only numbers between 10 and 18 that are not divisible by 2 or 3, broods of prime-period cicadas frequently escape high predation levels and hence tend to dominate hypothetical cicadas with nonprime periods. This mechanism for generating prime numbers relies on either externally driven two- and three-year cycles of predators, or predators that have strict fecundity schedules creating dynamics that tend to show two- or three-year oscillations.
Lehmann-Ziebart and his undergrads suspect that the cicadas emerge in these odd patterns as a balance between predator satiation and competition within the cicadas themselves. They, unfortunately, couldn’t find an obvious explanation for the prime numbers, though suggest it could have to do with a genetic counting mechanism.
Another explanation for this odd pattern of bird decreases coinciding with great cicada food sources is shoddy data. The Bird Breeding Survey is performed by mere citizens after all — can’t trust them! JUST KIDDING! There have been criticisms of the survey, including new observer bias (a n00b bird counter gets better each year, so the early years are unreliable and this bias is not accounted for), its road-based sites and transects for the counter’s ease could cause bias due to car traffic, and variation in the number of counters year-to-year. Nonetheless, the survey covers an incredible amount of ground — with each route covering nearly 25 miles — and over 400 bird species and has been ongoing since 1966. Pretty good for any large-scale data set which will always have caveats.
It looks like the jury’s still out on why bird populations decline during cicada emergences, though I suspect it’s a combination of many factors — bird communication problems, detectability bias, ecosystem changes induced by the cicadas and the overall variability in bird populations and routes.
(1) Wheeler, G., Williams, K., & Smith, K. (1992). Role of periodical cicadas (Homoptera: Cicadidae: Magicicada) in forest nutrient cycles Forest Ecology and Management, 51 (4), 339-346 DOI: 10.1016/0378-1127(92)90333-5
(2) Pray, C., Nowlin, W., & Vanni, M. (2009). Deposition and decomposition of periodical cicadas (Homoptera: Cicadidae: Magicicada) in woodland aquatic ecosystems Journal of the North American Benthological Society, 28 (1), 181-195 DOI: 10.1899/08-038.1
(3) Yang, L. (2004). Periodical Cicadas as Resource Pulses in North American Forests Science, 306 (5701), 1565-1567 DOI: 10.1126/science.1103114
(4) Yang, L. (2005). Interactions between a detrital resource pulse and a detritivore community Oecologia, 147 (3), 522-532 DOI: 10.1007/s00442-005-0276-0
(5) Speer, J., Clay, K., Bishop, G., & Creech, M. (2010). The Effect of Periodical Cicadas on Growth of Five Tree Species in Midwestern Deciduous Forests The American Midland Naturalist, 164 (2), 173-186 DOI: 10.1674/0003-0031-164.2.173
(6) Koenig, W., & Liebhold, A. (2003). Regional impacts of periodical cicadas on oak radial increment Canadian Journal of Forest Research, 33 (6), 1084-1089 DOI: 10.1139/X03-037
(7) Yang, L. (2008). PULSES OF DEAD PERIODICAL CICADAS INCREASE HERBIVORY OF AMERICAN BELLFLOWERS Ecology, 89 (6), 1497-1502 DOI: 10.1890/07-1853.1
(8) Krohne, D., Couillard, T., & Riddle, J. (1991). Population Responses of Peromyscus leucopus and Blarina brevicauda to Emergence of Periodical Cicadas American Midland Naturalist, 126 (2) DOI: 10.2307/2426107
(9) Koenig, W., & Liebhold, A. (2005). EFFECTS OF PERIODICAL CICADA EMERGENCES ON ABUNDANCE AND SYNCHRONY OF AVIAN POPULATIONS Ecology, 86 (7), 1873-1882 DOI: 10.1890/04-1175
(10) Koenig, W., Ries, L., Olsen, V., & Liebhold, A. (2011). Avian predators are less abundant during periodical cicada emergences, but why? Ecology, 92 (3), 784-790 DOI: 10.1890/10-1583.1
(11) Lehmann-Ziebarth, N., Heideman, P., Shapiro, R., Stoddart, S., Hsiao, C., Stephenson, G., Milewski, P., & Ives, A. (2005). EVOLUTION OF PERIODICITY IN PERIODICAL CICADAS Ecology, 86 (12), 3200-3211 DOI: 10.1890/04-1615