photograph of a White-crowned Sparrow by Rohan Kamath
muscles and jeweled adornments are nonarbitrary symbols in
human society, denoting strength and wealth, respectively.
On the other hand, many arbitrary symbols -- the bishop's
mitre, the admiral's gold braid, the karate master's black
belt, the judge's gown, the knight's title of Sir -- are
also recognized. Such symbols are arbitrary because they
signal status without having any inherent connection with
the status signaled.
Nonarbitrary status symbols are readily found in nonhuman animals, the classic example being the size of horns in mountain sheep. In those creatures a small-horned male will avoid a large-homed stranger seen at a distance, even though the two have
determined their relative positions in a dominance hierarchy
by actual combat. The existence of arbitrary symbols outside
of human society has been more problematic. British
evolutionists Richard Dawkins and John Krebs adopted the
term "badge" for arbitrary animal symbols, and avian
biologists, especially Sievert Rohwer, have carried out
investigations to see if badges play a role in dominance
relationships in bird flocks.
Harris' Sparrows assemble in flocks of mixed ages in the winter. Individuals within the flocks show a great deal of variation in their plumage characteristics, especially in the amount of darkness on their heads and "bibs." In a series of experiments Rohwer darkened relatively light first-year birds with dye to make them look like adults. He found, for example, that dyed first-year birds are initially avoided by undyed "control" first-year birds. Then the dyed birds began actively to dominate the controls. Rohwer thus concluded that the dominance status of a Harris' Sparrow can be communicated by a badge, a dark head and bib. The alternate explanation (that the dye in some way actually enhanced the combat ability of dyed birds) is too unlikely to merit serious consideration.
Differences in darkness do not, however, always correlate with status, as behaviorist Doris Watt has shown. In experiments Watt showed that within age classes, at least in small groups, darker birds are not always dominant over lighter individuals. She believes the darkness to be basically a badge of age, which signals to first-year birds the potential dominance of adults. Apparently among both young and adults, variation in breast spot patterns (as opposed to overall breast darkness) aids individuals in recognizing one another, but does not indicate status.
Status signals may allow associations to form between dominant and subordinate individuals so that both may benefit, as Rohwer and his colleague, Paul Ewald, have suggested. Subordinates, for instance, may learn what foods are most nutritious, while dominants may be able to place subordinates between themselves and potential predators.
Badges also exist in White-crowned Sparrows, as Gary Fugle and his coworkers at the University of California have demonstrated. Adult males, which have bright black and white striping on the head, have the highest dominance status, juvenile females with dull striping the lowest, and adult females with intermediate striping intermediate status. Experiments in which the heads of juvenile and adult females were painted to resemble adult males revealed that brighter striping added to the status of the painted birds relative to unpainted controls. Similarly, experiments with stuffed birds (mounts) indicate that a male Yellow Warbler signals his status, in particular his level of aggressive motivation, by the amount of brown streaking on his breast. Brighter (more streaked) mounts, elicited more aggressive responses from males than duller mounts, while brighter males responded more aggressively to mounts than did duller males.
Perhaps the most ingenious experiment on avian badges involved the European Great Tit, a close relative of North American chickadees and titmice (all are members of the same genus, Parus). This species has a dark cap, white cheeks, and a dark breast stripe/bib. Torbiorn Järvi and Marten Bakken of Norway's University of Trondheim used radio-controlled motorized stuffed birds at a feeder to test the efficacy of the breast stripe as a badge. When a Great Tit approached the feeder, the stuffed bird could be rotated to face the incoming individual and to perform a "head-up" aggressive display. When, and only when, the stuffed bird had a breast stripe wider than that of the incoming bird was the latter frightened away.
Evidently badges can signal status in avian societies -- but this presents evolutionists with a considerable mystery. Since they are arbitrary symbols, why isn't cheating widespread? Why don't first-year Harris' Sparrows grow dark plumage just like adults? Why don't weakling Great Tits develop broad breast stripes and monopolize the food at feeders? Badge systems seem to have built into them the seeds of their own evolutionary destruction, since frequent cheating should soon make all the signals ambiguous.
One way out of this seeming dilemma has been suggested. Perhaps dominance in monogamous birds (as opposed to polygynous ones, where dominant males may get many more matings) does not in itself confer a selective advantage. Subordinate birds may not get access to the best resources, but they also may not have to expend much energy defending what they've got. Being subordinate may be just as good an evolutionary strategy as being dominant -- both kinds of individual may be equally successful reproductively. If that is the case, there would be no advantage to cheating, and one would not expect such behavior to evolve.
Redwing Coverable Badges;
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.