If you should ever find yourself at Exeter St David’s railway station in Devon, England, pay close attention to what happens when someone flicks away a burning cigarette. While it may very well get stepped on, kicked onto the tracks, or otherwise snuffed out, if you are at the station during a quiet period you may just see something phenomenal; birds that fumigate themselves with the used butts. In an article issued a few weeks ago in the Telegraph, Rooks (Corvus frugilegus) were observed picking up still-smoldering cigarettes, moving them to a more private location, and opening up their wings to collect the smoke. The intelligence of Corvid birds (crows and their relatives) is well-known, although this is perhaps the first time that these birds have been seen taking advantage of discarded suicide sticks to benefit themselves, probably in an attempt to kill parasites living on their wings.
When my friend Molly first sent me the article (hat-tip to Molly) a few weeks ago, the first thing that came to my mind was the behavior known as “anting.” Although I had never studied it in detail, I remembered that some birds pick up ants and rub them on their feathers, the formic acid the ants secrete in an attempt to defend themselves having a detrimental effect on the parasites living on the bird (as suggested by Kelso and Nice’s “A Russian Contribution to Anting and Feather Mites“). In fact, the first mental image was of a crow, holding a smoking cigarette in its beak, rubbing the ashes over its wings. As the article makes clear, however, this is not the case and it is not merely an example of the birds doing an old behavior with a new tool; they have done something that appears to be entirely new.
Still, I thought I would look into anting behavior a bit more; the benefits to birds that exhibit the behavior are clear, but stopping inquiry there would be rather adaptationist, claiming that birds do it because it benefits them, but why birds engage in this behavior at all might not be as straightforward as it seems. In a 2004 paper entitled “Bactericidal and Fungicidal Activity of Ant Chemicals on Feather Parasites: An Evaluation of Anting Behavior,” Revis and Waller found that the bacteria Bacillus licheniformis, Bacillus subtilis, and fungi Chaetomium globosum, Penicillium chrysogenum, and Trichoderma viride were not inhibited by the natural concentrations of chemicals excreted by several species of ants (although concentrated formic acid did work), suggesting that the ants selected simply did not contain concentrated-enough doses of formic acid to provide the birds protection against the bacterial and fungi tested in the study. This is surprising, especially since tests don’t seem to support the idea that anting inhibits arthropod parasites either, yet the birds keep on exhibiting the behavior. Why?
What is often left out when anting is discussed is that the birds often eat the ants, supposedly getting a “two-for-one” kind of benefit from picking up ants. This could give us a clue to the real motive behind anting, however, and although I do not have access to the article, Judson and Bennett (1992) hypothesized that anting may actually serve to remove toxic formic acid from ants prior to ingestion. From the abstract to their paper “‘Anting’ as food preparation: formic acid is worse on an empty stomach“;
Anting is a behavior common among passerine birds, yet its function is unknown. The behavior consists of a highly stereotyped set of movements which start when a bird picks up an ant, usually one which sprays formic acid as a defense, and sweeps it with frenzied motions through its feathers. The bird will often also eat the ant. As formic acid is toxic, we have tested the food-preparation hypothesis, that is, that the birds are anting to remove a distasteful or toxic substance from the ant before eating it. In a pair of experiments on starlings, Sturnus vulgaris, we have found evidence in support of this hypothesis.
[Again, I don’t have access, but for information on the toxicity of formic acid to birds see Bennett, Llloyd, and Cuthill’s 1996 paper “Ant-derived formic acid can be toxic for birds.” It should also be noted that it is sometimes difficult to tell whether some birds eat or discard ants after using them in anting behavior, as described in Osborn’s “Anting by an American Dipper (Cinclus mexicanus)]
Indeed, anting behavior doesn’t always include ants, and the observations of birds “anting” with various other insects seems to support the notion that the birds are ridding their prey of noxious chemicals rather than trying to kill their parasites. One such example is described in Eisner, et. al’s “Pre-ingestive treatment of bombardier beetles by jays: food preparation by ‘anting’ and ‘sand-wiping’,” where Blue Jays subject bombardier beetles to anting prior to ingestion, making sure that they don’t get a stomach full of toxic chemicals. The study notes that the jays used in the tests did not always “ant” with the beetles, however; sometimes the beetles were repeatedly pecked, dropped, and picked up again before being consumed (“pre-milked” beetles were pecked and eaten quickly, so the “peck until the beetle is done discharging” seems like the primary way of handling the beetles). Snails, berries (VanderWerf, 2005), and caterpillars (Wenny, 1998) have also been observed being used during anting by birds, but in each of these three cases the prey was not eaten (or did not appear to be in the case of the caterpillar). These bouts of anting were followed by preening, suggesting that there perhaps there is some benefit to the chemicals present in all three organisms to the bird, although we cannot be sure of what. The incident that VanderWerf is even more interesting as the event was witnessed on one of the Hawaiian islands, where ants, the snail, and the berry are not native and the bird (the ‘Elepaio or Chasiempis sandwichensis) is non-migratory. This shows that even though ants are most commonly associated with anting, the behavior can incorporate a variety of different resources and is not restricted to species that come into contact with ants on a regular basis.
Animals other than birds exhibit anting behavior, however; just this year Verderane, et al. published “Anting in a Semifree-ranging Group of Cebus apella,” describing how one particular species of Capuchin monkey rubs ants all over its body, a behavior that correlates with the seasonal incidence of an ectoparasite. This, the researchers conclude, is true anting and actually does work on the pests in the primate’s fur, whereas the benefits of anting in birds (where it was first described) is still a bit of a mystery.
Part of the reason why anting is such a mystery is that it involves interactions between hundreds of species all over the world; I don’t know if it’s even possible (or wise) to impose an “always/never” rule on what benefits the birds may or may not be receiving from anting. While the evolution of the behavior likely started by trying to make prey insects expel their noxious chemicals prior to ingestion, it could very well have been co-opted to take advantage of naturally-occurring substances present on other organisms that may positively affect the health of the bird. Depending on where you are, the strength of chemicals in the available ant/organism, and the weaknesses of your particular parasites, anting behavior could be used primarily in terms of food prep or it could indeed have a medicinal purpose; the amount of research into all the existing combinations is staggering.
Anting aside, what the English rooks appear to be doing is something that seems to be more straightforward; I really do think that they are trying to kill the parasites on their wings, although they likely only recognize the unpleasant feeling of being inhabited by parasites and a cessation of that feeling when the smoke their own wings. What I would be interested to see (other than an actual test of this hypothesis) is the potential harm done to the birds respiratory system by the cigarettes they use. While it is true that they receive some health benefits from removing parasites from their wings, are their lungs adversely affected by the smoke in a way similar to ours? This could be a great opportunity for scientists interested in health, behavior, and toxicology to collaborate (it reminds me of one of a study I would like to do, checking the health/cholesterol of animals found in theme parks which sell fried foods vs. the health/cholesterol levels of other populations in cities, suburbs, parks, and forests). Hopefully some research will be forthcoming, but until then I hope keen folks continue to keep an eye out for amazing bird behaviors wherever they are.