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Bower Bird Blues
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AMOROUS
ARCHITECTURE
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A female bower bird considers
her options.
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Scientists know that the amorous architects of the bird world build three
basic kinds of bowers: "maypoles," "mats," and "avenues."
But only now are they beginning to discover the reasons behind the existence
of these different forms. Mat, or platform, bowers are among the simplest:
thick pads of plant material ringed with ornaments. One mat-builder, Australia's
Tooth-billed catbird, builds what is known as a "circus ring"
by arranging silvery leaves around the mat, like the petals of a disheveled
flower. The bird constantly removes withered leaves in favor of fresh,
shiny replacements. The more ambitious maypole bowers are twig towers
built around one or a few saplings in a carefully groomed courtyard. The
Golden bower bird even perches on a roofed bridge suspended between towers.
And four other kinds of maypole builders surround their creations with
lawns of moss. Avenue bowers, such as the Satin bower bird's, featured
on NATURE, have two close-set parallel walls of sticks that sometimes
arch over to create a tunnel. In a rare example of a bird using a tool,
Satin and Regent bower birds may use a leaf or twig to paint the inner
walls of their bowers with a stain made from chewed plants, charcoal,
and saliva.
Gerald Borgia, a University of Maryland bower bird expert, believes the
different kinds of bowers all serve essentially the same function: to
make visiting female bower birds feel comfortable by protecting them from
overeager males. Courtship rituals, he notes, almost always involve males
and females standing with the bower between them, like a fence. In the
case of the maypole-building Macgregor's bower bird, for instance, the
courting pair warily circles the central tower. Only when the female chooses
to stop and allow the male to approach can mating occur. "The bower
probably started as a protective device," Borgia concludes. "It
allows females to get close enough to get a good look without feeling
threatened. The male that builds something that makes the females feel
most comfortable is likely to see more females." Borgia has also
detected a relationship between bower type and intensity of the male's
display. The male Spotted bower bird, for instance, builds a wide straw
wall and performs a relatively energetic display full of dance steps and
dramatic poses. In contrast, species building smaller barriers have toned-down
displays that are probably less threatening to females.
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A male bower bird checks
his handiwork.
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Other researchers have noticed a link between the showiness of a bower
bird's plumage and the intricacy of its bower: in general, the drabber
the bird, the fancier the bower will be. Some believe this reflects
an evolutionary choice: drab birds compensate for their dull appearances
by building flashier nests.
Borgia has also noticed that bower complexity sometimes varies with
topography. For instance, species living on hilltops build more modest
bowers than those living in valleys. The explanation, he says, may be
the amount of light that penetrates the forest in the two kinds of habitat.
Ridge tops are often shrouded in clouds, allowing only dim light. Hence,
to best show off their decorations, bower birds living here may build
more open bowers to make best use of available light. In contrast, light
is less of an issue in the valleys, so bower birds can afford to have
more elaborate roofed structures.
Surprisingly, Borgia has found that bower birds that build similar-shaped
bowers aren't necessarily closely related to each other. Using a DNA
fingerprinting technique, he and his colleagues drew a family tree for
bower birds that showed their evolutionary relationships. It suggested
that species that evolved at different times have independently learned
to build similar kinds of bowers, possibly because they faced similar
kinds of environmental conditions.
But close observation can reveal important differences in the seemingly-similar
structures, Borgia says. Where one species may build its bower from
the bottom up, for instance, the other may start a similar structure
at the top and build down. Similarly, some species put the entrance
to their bowers on the uphill side, while similar structures built by
other species face downhill. Nobody knows whether young bower birds
learn such practices from their elders, or whether they are encoded
in their genes at birth. It is a much-debated question that Borgia hopes
to answer in future studies by rearing native males in captivity with
and without mature tutors.
Bower Bird Blues
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