It
all started with hens. Norwegian scientist Thorleif
Schjelderup-Ebbe wondered how peace was kept in their
flocks, and conducted a series of experiments to find out.
He discovered that things were tranquil only in established
flocks -- ones in which each hen knew its place. And the
hens learned their places in fights over chicken feed. Once
a hen had been bested in a squabble, it henceforth would
defer to the victor. Each hen knew whom it could dominate,
and by whom it would be dominated. A "peck order" was thus
established in the flock and functioned to maintain social
stability. Hens can recognize many other hens and can
remember their dominance status relative to each. One hen
demonstrated the ability to recognize 27 other individuals
belonging to four different flocks. As you would expect, the
birds at the top of the peck order benefited both by
increased access to food and by avoidance of injuries (even
bullies can get hurt in fights). The birds at the bottom,
while having to wait until those higher up had eaten their
fill, at least were not subjected to continuous fights that
they were likely to lose. In all but polyandrous
species of birds (those where one female mates with more
than one male), males are normally dominant over females.
That is especially true in the early stages of pair
formation, although in some buntings and other finches and
in some gulls, a reversal of dominance, with the male
becoming subordinate, reportedly occurs after the pair bond
is established. In experiments, females have been able to
assume higher ranks in dominance hierarchies after receiving
injections of male hormones and by dyeing their feathers to
resemble male plumage. Results of such experiments varied
from species to species; doses of male hormone increased the
social status of female European Chaffinches and Japanese
Quail, but not of starlings. It is usually assumed that a
high position in a dominance hierarchy increases the chances
of survival and also increases reproductive output. In
species where males display on leks, the dominant male
generally holds the best territory on the lek and
successfully copulates with the most females. But there is
relatively little evidence in monogamous species that
dominant individuals have a better chance of survival than
subordinates. A study of juvenile Song
Sparrows on an island near Victoria, British Columbia,
showed that dominance relationships at millet-provisioned
feeders during summer were reflected in chances for
surviving the subsequent winter and establishing a territory
the following spring. In two consecutive years, dominant
males showed 35 and 22 percent better survival than
subordinate males. For dominant females the equivalent
figures were 32 and 33 percent. The effects on successful
settlement of territories were similar for both sexes --
dominant individuals were more likely to settle into
territories than subordinates. The same study showed that
sex and age were the major determinants of dominance. Males
more frequently dominated females than vice versa; males
tended to win about 60 percent of their encounters with
females at the feeders, females about 40 percent. Time since
hatching had a strong effect on the success of encounters,
even when only 24 days separated the oldest juvenile from
the youngest. In contrast, no effect of body size on
dominance was detected; a small bird was as likely to
dominate a larger one as the reverse. Overall, it appeared
that next after sex and age, previous experience in
encounters was the most important factor in achieving
dominance. Thus, early nesting would appear to be the best
reproductive strategy for Song Sparrows. Young produced
early in the season will gain the most experience and are
the most likely to survive to reproduce. But if nesting is
started too early, offspring may be killed by food shortage
or late winter storms. Undoubtedly, there is a fine
ecological line between reaping the advantages of early
nesting and suffering the consequences of premature
breeding. In another study of Yellow
Warblers, however, Michael Studd and Raleigh Robertson of
Queens University in Ontario discovered that the nestlings
of subordinate birds grew just as fast as those of dominant
birds. At least by that measure (to be sure, not an
exhaustive one), natural selection was not operating in
favor of dominant birds. It has been suggested that in many
circumstances being dominant is no more advantageous than
being subordinate. For example, a dominant male may obtain a
territory which is more resource-rich than that of a
subordinate, but it may have to spend much more energy to
protect it. Therefore, costs and benefits may more or less
balance one another at each level in a dominance hierarchy,
and evolution may favor the maintenance of the hierarchy
itself, rather than just those near the top (which could
lead to the disappearance of differences in dominance). Such
a situation in which selection leads to the coexistence of
several different modes of behavior has been called by
British evolutionist John Maynard Smith a "mixed
evolutionarily stable strategy." SEE: Bird
Badges;
Natural
Selection;
Visual
Displays. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.