Among Bowerbirds, Mimicry Wins Mates, Study Says

Bijal P. Trivedi
National Geographic Today

September 22, 2003

Male bowerbirds famously woo females by fashioning elaborate
bowers—not nests but U-shaped showplaces with parallel walls of
twigs. There the males prance and noisily serenade the females.

This spectacular courtship ritual during the November and December mating season has long attracted ornithologists to the species formally known as satin bowerbirds, native to Australia.

New research, however, reveals that the ritual may be even more complex and subtle than previously recognized; male satin bowerbirds mimic the calls of other bird species to attract a mate.

Understanding the dynamic of bowerbird courtship helps scientists understand sexual selection—a hot topic since Charles Darwin first proposed it in 1871. Darwin observed that females were attracted to "fancy males" who displayed traits that appealed to the eye and ear—purely, he hypothesized, for the purpose of sexual attraction.

Once the males have built their bowers, they perch in them, squawk and whistle.

"The males are basically saying, I'm here in my bower, I'm available and I'm ready for assessment," says Seth Coleman, a doctoral candidate in behavioral ecology at the University of Maryland in College Park who has led a three-year bowerbird research project in the Australian bush, funded by the National Science Foundation.

When a female enters a bower, the male ruffles his feathers, stretches his wings and makes loud buzzing sounds as he struts and runs about. Meanwhile the female also is apparently checking out his architectural talent—the bower's symmetry and decoration (females are partial to anything blue like a feather, a bottle cap or a shell).

Mimicking Five Species

Previous studies have shown that females are less likely to choose a male in an unkempt bower.

But architecture isn't everything. Last year, Gail Patricelli, a former colleague and lab partner of Coleman who is now with the Cornell Lab of Ornithology in Ithaca, N.Y., discovered that successful males could interpret the females' body language and modify their courtship moves accordingly.

After the male's raucous vocalizations, he makes a more intimate appeal, coming almost beak to beak with the female and softly mimicking five other Australian birds: the kookaburra, the Lewin's honeyeater, the sulfur-crested cockatoo, a Torresian crow and a yellow tailed black cockatoo.

If the mimicry impresses the female, mating proceeds apace. Otherwise she flies to the next bower.

"This suggests that mimicry is pretty important in the final decision of mate choice," Coleman says. He described his results at the Conference on Acoustic Communication by Animals, hosted by the University of Maryland in July.

Among bowerbirds, "(the mating system) is like a singles bar," says Jack Bradbury, a behavioral ecologist at the Cornell Lab of Ornithology. Males want to mate as widely as possible to spread their genes. But females, who do the parenting, need to discriminate for mates of good genetic stock—helping ensure survival of the offspring.

Coleman carried out his field work at Wallaby Creek, about 90 miles southwest of Brisbane, Australia, within a 3 square mile (8 square kilometer) plot of forest in Tooloom National Park. More than 250 species of birds inhabit the park, a favorite of birders.

Good Mimics, Many Mates

Coleman rigged up 35 motion-sensitive cameras and microphones—one per bower. Each research season—from September through December—the cameras recorded about 4,000 hours of bowerbird activities.

Back in the lab, Coleman analyzed the vocalizations using a software program called Canary, developed by the Cornell Lab. Canary creates a spectrogram, or visual representation of each bird call.

Coleman produced spectrograms for each of the bowerbird's mimicked calls, then compared them with spectrograms for the model species.

The research shows that the best mimics with the broadest repertoire were most successful at wooing the females. The top males mimicked four or five species with accuracy and mated with 25 different females in one season, according to Coleman.

A poor mimic could only imitate the vocalizations of one or two species.

"(A poor mimic) sounds like a kookaburra with bronchitis," says Coleman, "and doesn't mate with a single female. These guys are probably lifelong losers."

The quality of mimicry may reveal something about a general ability to learn, Coleman speculates.

"Females need a way of getting honest information about a potential mate," says Bradbury. "Some species engage in courtship feeding, which tests the males' ability to gather food. Canaries, which sing incredibly complicated songs, can only do so when they are in good shape."

Stephen Nowicki, an evolutionary ecologist at Duke University in Durham, N.C., has proposed that what females really want in a male is a good brain. He and his colleagues have shown that when song sparrows get less food when young, their brain is smaller, inhibiting their ability to learn songs—and to lure mates.

"Seth's work is significant because he has identified yet another of these 'fancy traits' that females use to select a mate," Nowicki says.

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