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. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.