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Murray Gell-MannThe Quark and the Jaguar258 |
Deception Among Birds
For amusing examples of the exploitation of opportunities by species interacting with other species, we can turn to lying as practiced by animals other than humans. Deception by mimicry is well known; the viceroy butterfly, for instance, resembles the monarch and thus profits by the bad taste of the latter. The cuckoo (in the Old World) and the cowbird (in the New World) practice another kind of deception by lazing their eggs in the nests of other birds; the intrusive chicks then do away with the eggs or chicks that belong in the nest and monopolize the attention of the foster parents. But actual lying?
We are accumstomed to hearing people lie, but it is somehow more surprising in other organisms. When the Argentine Navy spots a mysterious periscope in the estuary of the Río de la Plata just before the budgets of the armed forces are to be considered by the legislature, we suspect that deception is being practiced so as to capture additional resources, and we are not particularly astonished. But the analogous behaviour among birds is more unexpected.
One such case was discovered recently by my friend Charles Munn, an ornithologist studying mixed feeding flocks in the lowland tropical forest of Manu National Park in Peru. Some species forage together in the understory or lower canopy of the forest and others in the middle canopy, where they are sometimes joined by colorful fruit-eating tangers from the upper canopy. (Amon the species found in those flocks in winter are a few North American migrants. Further north in South and Central America there are many more. We residents of North America know them as nesting species in the summer and are intrigued to find them leading a very different life in a distant land. If they are to return zear after year to nest, their habitats in the southern countries will be jeopardized if North American forests are chopped up into still smaller parcels than the ones now remaining. For one thing, thinning out the forests permits further inroads bei parasitic cowbirds.)
In each mixed feeding flock, there are one or two sentinel species, which move about in such a way that they are usually near the center of the flock or just below. The sentinels warn the others by a special call of approaching bird that might turn out to be raptors. Charlie noticed that the sentinels for the understory flocks sometimes gave the warning signals even when no danger was apparent. Looking more closely, he found that the fake alarm often permitted the sentinel to grab a succulent morsel that another member of the flock might otherwise have eaten. Careful observation revealed that that the sentinels were practicing deception about 15 percent of the time and often profiting by it. Wondering if the phenomenon might be more general, Charlie examined the behavior of the middle canopy flocks and found the sentinels there are doing the same thing. For the two species of sentinels, the percentage of false signals was about the same. Presumably, if the percentage were much higher, the signals would not be accepted by the rest of the flock (recall the story The Boy Who Cried “Wolf”), and if it were much lower, the opportunity for the sentinel to obtain extra food by lying would be partially or wholly wasted. I am intrigued by the challenge of deriving by some kind of mathematical resoning the figure of about 15 percent; in a plausible model, might it come out one divided by two pi? When I asked that question of Charles Bennett, he was reminded of something his father had told him about the Royal Canadian Air Force units based in England during the Second World War. They found it useful, when sending out a fighter and a bomber together, to attempt occasionally to deceive the Luftwaffe by positioning the fighter below the bomber rather than above. After a good deal of trial and error, they ended up following that practice at random one time in seven.