Sexy secrets of birds’ signals

By Geoff Maslen
This article was originally published in University World News on 17 October 2010

Thanh-Lan Gluckman’s discovery that birds have sexy secrets, never divulged before, has startled zoologists around the world. Then again, it’s not often a masters student in Melbourne uncovers something that not only changes zoologists’ fundamental beliefs about sexual communication among birds but which also gets her into a PhD programme at Cambridge University.

Yet Gluckman has done just that after publishing her research results last month as a paper in the prestigious Journal of Evolutionary Biology.

Her discovery, that feathers need not be as bright and showy as a peacock’s to be used in sexual signalling, overturns a widely accepted notion among biologists that patterns on a bird’s feathers are just for camouflage. Instead, Gluckman found they can also have an important role in attracting a mate or in competition with other males.

A masters of philosophy student in zoology at the University of Melbourne, Gluckman undertook the research with her former lecturer, Dr Gonçalo Cardoso, who is now at the Research Centre in Biodiversity and Genetic Resources in Portugal.

During her honours year she helped Cardoso develop a software-based method to investigate how barred patterns in bird plumage might function as a form of communication. She suggested comparing barred plumage in juveniles and adults and Cardoso agreed she should start the project as a research assistant – a week after handing in her honours thesis.

“It’s really amazing when someone at that level recognises your ideas and allows you the freedom to continue exploring. It can quite quickly lead to obsession, as it has with me, and I haven’t looked back since,” she says.

Hundreds of bird species, among them Australia’s zebra finches and budgerigars, have ‘barred’ patterns on their bodies with alternating dark and light pigmentation side by side. In the zebra finch, barred patterns appear only in males in adulthood while the budgerigar has the patterns on the back of males, females and juveniles.

“Since Darwin wrote of visual communication in birds, we have known that bright-coloured feathers play a role in sexual signalling, for example to attract females,” Gluckman says. “But the role of barred patterns as a communication signal has largely been overlooked.”

Her study involved a large-scale comparison of plumage patterns of nearly 9,000 bird species around the world, 90% of the total. Using field guides of birds covering Earth’s 17 major land masses, she compared barred plumage and other patterns on the bodies of the males, females and juveniles to assess what they might be used for.

Compiling what she thought would be all the data needed took her months of working up to15 hours a day, seven days a week. Although the research involved checking thousands upon thousands of photographs and drawings of birds and recording their details, she calls it “a labour of love” – and this despite Cardoso’s early warning it would become arduous after the first 1,000 species.

But a shock was waiting for her: “I thought I had finally finished my data collecting and was feeling very pleased with myself. So I gave the data to Gonçalo and went on a holiday to the Amazon. On my first day back from holidays, he told me, ‘That’s very, very good…but you missed a bit’ and I had to go back over all the data again!”

The crucial ‘bit’ she missed was recording if there were illustrations for each juvenile bird. This was important because the researchers could only use the data on birds whose juvenile pattern had been described.

“That took about another six weeks going back through all the books, although I was faster the second time round. What we were analysing related to Darwin’s early work on sexual traits being biased towards adult males,” Gluckman says.

“If barred plumage patterns were to function as a form of communication, rather than just as camouflage, they would generally be a male attribute and would appear at sexual maturity. That’s why it was important to know what patterns juvenile birds have.”

Although she found evidence that barred plumage was predominantly used as camouflage, she also discovered that compared with other feather patterns it was much more likely to appear only in males and only at sexual maturity.

“We found these differences between the sexes on the front of the birds which, of course, is an important area for communication during face-to-face interactions. But we did not find any differences [in the barred patterns] on the back of the birds which is useful for camouflage when incubating eggs or hiding from predators,” Gluckman says.

“The really exciting finding was that communication patterns do not have to be showy or bright, like the bold iridescence of peacock plumage, but could assist in camouflage and still function as a sexual signal. This changes our understanding of communication in birds and of avian visual ecology.”

From a distance, barred plumage patterns confer a camouflage advantage because, although contrasting dark and light bars are visually stimulating at close range, to a predator at a distance the patterns would probably merge and be perceived as a single colour.

At close range, however, the patterns might act as a communication signal. Gluckman says the alignment of barred patterns across feathers shows the quality of the bird’s plumage and how well maintained it is: “If you were a bird that would be worth showing off in a social context.”

The point here, Gluckman says, is that if the male’s feathers are not particularly well maintained, or if the bird is not healthy, the pattern may be irregular or reveal patches in the plumage.

“But if it is a healthy individual, the patterns may be more regular. So the pattern draws attention to how healthy the bird is – or it may help to make the male more ‘sexy’ in some way.”

She says behavioural studies are needed to find out if this is the case. The experiment would involve modifying the plumage of different male birds and then assessing the female’s response in terms of her choice of mate.

Of course, how a human sees plumage patterns might appear differently to a bird. Gluckman says birds have a different visual acuity from us: they have three types of cones in their eyes and can see in the ultraviolet and violet range as well as in the normal red to blue range of humans. So barred patterns might appear quite different to the birds themselves.

“Aside from direct behavioural studies, we also need to research how birds see the patterns to really nail down what happens from their perspective. This is now possible because of the technology and research that have come about in the past couple of decades.”

A descendant of a family whose origins date back to the Russian Jewish people who migrated to South Africa before the Boer war, Gluckman says her surname, although German, was an administrative error at customs and has stuck ever since.

Her father is South African and her mother is Vietnamese and, to add to the geographical mix, her parents met in Singapore but she was born in France where many Vietnamese lived after the Vietnam War.

Her family later moved to Melbourne where she began her studies at Melbourne University. Now she plans to continue her investigations next year as part of her PhD at Cambridge: “I will be working with Dr Martin Stevens who has studied visual systems in birds and with Professor Nick Davies, one of the founding fathers of this area of research – behavioural ecology.”