DO PARROTS GET THEIR COLORS?
by Whit Gibbons
September 25, 2005
year, the rediscovery of the ivory-billed woodpecker, which was thought
to be extinct, surprised ornithologists everywhere. But an equally important
finding from a scientific perspective was research that provided a new
explanation for why male and female birds may differ dramatically in plumage
always been noted for sexual dichromatism, with males characteristically
being the more brightly colored. Sexual dichromatism is an easy scientific
term to remember. “Di” means “double,” and “chromatism”
refers to color, so a species with sexual dichromatism is one in which
the two sexes differ in color. Sexual dichromatism is particularly apparent
in birds. Male goldfinches turn bright yellow in the spring, whereas females
maintain a subdued, olive coloration. Male cardinals are much brighter
red than females. And even in the common house sparrow the male’s
suit of brown is more vivid than that of the drab female.
are particularly dramatic in most species of tropical parrots, with brightly
plumaged males and less colorful females. A study this year examined an
Australian parrot in which the opposite is true. The investigators discovered
why the two sexes differ in color but in a manner different from that
of other parrots.
colored red and blue females of the Australian parrot are so distinctly
different in appearance from the duller green males that early ornithologists
classified them as separate species. Reversed sexual dichromatism is known
among birds, but when females are more colorful than males, the species
typically has sex role reversal also. In other words, with all previously
known cases, females compete with each other for male mates, instead of
the conventional situation in which males compete with males. In addition,
the males care for the eggs and young. Not so with the Australian parrot.
Despite the disparate color patterns between the sexes, Autralian parrot
males still compete for mates and females tend the eggs and young, just
like with most birds in which the males are the brightly colored ones.
part of the field of ecology is that you are faced with intriguing biological
puzzles every day. In this case, the investigators had a dilemma in which
an observed phenomenon of nature did not fit the traditional model. These
male parrots, which were not nearly as colorful as the females, should
have been assuming a more motherly role. So what made them different form
all the other examples in the bird world?
the answer lies in the evolutionary history of the species, and in the
pressures of natural selection related to ecology and lifestyle, which
operate independently in the two sexes of the parrot. One way the two
sexes differ is that the females live most of the year in the forest in
tree hollows where they also nest. They forage near the hollows, to which
they retreat from predators. Equally as important as escaping predators
is that they must protect their young. And it turns out that one of the
greatest threats to babies in the nest is other female parrots, which
will actually come into the nest hollow and kill the offspring. In defending
the hollow and nest from other parrots, the female’s bright plumage
makes her more conspicuous and a more threatening competitor. So, natural
selection has favored bright plumage in the females.
males have been selected for green plumage, which makes them less conspicuous
to aerial predators against leaves in the tree canopy where they spend
much of their time foraging, with no tree hollows to retreat to for escape
from raptors. However, the green color makes them highly visible to other
males against tree trunks below the canopy where the females are to be
found during the breeding season and where competition for mates occurs
between males. The study demonstrates how evolution can lead to exceedingly
complex scenarios in nature and specifically how plumage color can evolve
independently in two sexes of a species.
By the way,
in case you ever have a chance to observe one, ivory-billed woodpeckers
show sexual dichromatism. Both sexes are contrasting black and white,
and both have a large crest on the head, but the crest of the female is
black and that of the male is bright red.
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