Redwing Coverable Badges

A corporal can tell that another soldier is a sergeant by the number of stripes on the sergeant's sleeve; we are informed of the status of a plainclothes detective when he flashes his shield at us. Badges play significant roles in human societies. Do they also serve important functions in avian societies?

In nonhuman animals, "badges" may simply be defined as arbitrary visual cues, often taking the form of especially prominent patches of color, that signal social status. One of the most familiar badges in the bird world is the bright red epaulette of the Red-winged Blackbird. Rare indeed is the birder who has not enjoyed the spectacle of a male Redwing, perched on a reed, singing while flashing forward the brilliant patches that give him his name.

What is the function of the Redwing's badge; does it serve to distinguish Redwings from other blackbirds, or does it signal status within Redwing society? That the epaulettes are important within the Redwing social system was suggested by experiments in which the patches of adult males were dyed black. Such males had much more difficulty holding their territories than "control" males with unmodified red epaulettes. In two separate experiments, over 60 percent of the blackened males lost their territories; less than 10 percent of the control males were evicted. These results, however, do not definitively answer the question of whether the badges function as intraspecific or interspecific signals. It could be that the epaulettes are species-recognition signals, and that other male Redwings did not realize that the dyed males were members of their own species. Since no other Redwings appeared to be present, male Redwings may have continued intruding into the dyed males' territories until the latter became exhausted and gave up the defense.

Experiments with mounted dead birds (mounts) by Andrew Hansen and Sievert Rohwer allowed the "social status" and "species recognition" hypotheses to be distinguished. Territorial male Redwings responded to mounts by approaching or attacking them or by displaying to them. The researchers found that the males responded more strongly to Redwing mounts with their epaulettes darkened than to mounts of male Brewer's Blackbirds, a species that shares the Redwing habitat and whose males resemble the dyed Redwings. In fact, the Brewer's mounts were largely ignored. The Redwings' ability to discriminate between the two species in the absence of the badge indicates that the epaulette is not necessary for species recognition and supports the social status hypothesis.

Hansen and Rohwer further tested and expanded that hypothesis. They recorded the responses of territorial males to Redwing mounts that had epaulettes totally blackened, half-blackened, normal, and twice as large as normal (the supplemental epaulettes were cut from Redwing study skins and glued behind normal ones on the mounts). The males invariably showed aggression that was proportional to epaulette size; some of the mounts with double epaulettes were violently attacked and suffered substantial damage.

This explains another of Hansen and Rohwer's observations -- that males intruding into occupied territories greatly limit the exposure of their epaulettes. While searching for territories of their own or seeking food within another male's territory, keeping one's epaulettes covered reduced the chances of being assaulted. When the experimenters removed territorial males, however, intruders into the newly empty territories at first kept their epaulettes covered, but within a few minutes began to expose them. Within about half an hour the intruders were displaying like owners. The time that elapsed before owner-like behavior appeared was roughly the same as the length of absences of owners from their territories in the ordinary course of events, time enough for the intruder to be reasonably certain the owner would not return.

All of this work supports what Hansen and Rohwer call the "coverable badge hypothesis." It assumes that Redwings (and other birds with badges that can be either displayed or covered, such as kinglets) benefit from being able to signal their intentions either to fight as owner of a territory or to depart submissively. They predict that coverable badges will evolve in territorial systems where (1) owners have a high probability of evicting intruders; (2) males frequently "trespass" in search of food or vacant territories; (3) fighting involves a high risk of injury to both combatants; and (4) adult males cannot predict whether they are going to be owners or "floaters" (males unable to establish territories). These conditions seem to be met in the Redwing system -- intruders presumably searching for territories conceal their badges and leave without a battle when owners display theirs.

So, when you watch Redwings, see if you can spot trespassers into territories, and record whether they normally cover their badges and depart when the owner displays at them or chases them. Hansen and Rohwer studied Redwings in Grant County, Washington. It would be interesting to ascertain whether their observations apply to other populations of these widely distributed birds. If you have the opportunity of studying Tricolored Blackbirds, try to determine whether they make the same use of their prominent epaulettes. And, of course, it would be fascinating to discover if kinglets employ their red and gold crowns in a similar manner.

SEE: Bird Badges; Visual Displays; Territoriality.
