ScienceWise - May/Jun 2006

Why parrots see red

Article Illustration
Male and female specimens of the Eclectus parrot. Their colouring is so different they were at first thought to be different species.
Article Illustration
Dr Rob Heinsohn measures the light coming off the parrot’s feathers.
Article Illustration
The parrot is only found in the tops of the tallest trees in the rainforest, so to study them requires good climbing skills.

The general principle in competitive sexual selection is that the sex that invests least in the rearing of offspring has the most energy to spare for beautification and display. Animals of the ‘drab’ sex select mates from the ‘beautiful’ sex based on their looks and competitive behaviour. This principle can be seen in many species of birds such as the Mallard duck where the male is brightly coloured while the female is dull brown. Of course, it’s not always the male that is beautified, but the general rule in bird beautification occurs in either one gender or the other, not both. The Eclectus parrot (Eclectus roratus), however, is an exception. Found in the rainforests of north eastern Australia and Papua New Guinea, the Eclectus parrot is unique in the animal kingdom as both the males and females are beautified in strikingly different ways. The two are so different that scientists thought them to be completely different species until they were seen mating.

Studying the Eclectus parrot offers scientist Dr Rob Heinsohn, from the Centre for Resource and Environmental Studies, a fascinating opportunity to explore evolution working in an unusual way. However, observing the parrot up close is not as straightforward as it sounds. As well as only being found in remote and inaccessible areas, these rare parrots live in hollows in the highest trees of the rainforest canopy and almost never visit the forest floor.

To study the parrots Dr Heinsohn had to spend months at a time living in a makeshift shelter in the remote rainforest. By day he would wander the dense forest floor with an expert ear tuned to the birds distinctive cry. Having located a nest, he would then don climbing gear, shoot a grappling line up the 100 metre tree and climb the enormous trunk. Once in the treetops, Dr Heinsohn was able to take blood samples from the chicks so that when back in the lab, he could perform DNA analysis to establish which birds were mating with which - essential information for biologist trying to understand sexual selection.

However, the mating habits of the birds are only half the story. The other important question was why the birds had evolved the particular display colour schemes in both males and females. To measure this in an objective manner, Dr Heinsohn used a spectrophotometer to map the wavelengths of light that the feathers reflect. This is an essential step because bird eyes are quite different to human eyes in that they see much further into the ultraviolet than we do. It’s important to know what one parrot looks like to another parrot.

After years of study and numerous gravity defying visits to the upper rainforest canopy, Dr Heinsohn and his co-workers were able to build up a fascinating picture of the life of these extraordinary birds.

Suitable nest hollows for the birds are very rare. The female birds are fiercely competitive over available sites.

A typical female Eclectus parrot spends as much as 11 months of the year at home defending her nest site from other females. Of course the only way she is able to do this is if a male bird will feed her, and the only way to persuade him to do that is to have his chicks - or at least make him believe the chicks are his. To achieve this, her plumage adaptation is a bright conspicuous red – attractive to males whilst aggressive to other females. Because she spends most of her time in the relative safety of the nest, her bright plumage doesn’t make her especially vulnerable to predators such as kites and hawks.

The male, on the other hand, has quite a different problem. He must fly far and wide to gather food for the female and their chicks, but he must also be able to compete for her attention and intimidate other males once back at the nest site. He is able to achieve this with some remarkable colour chemistry in his plumage. When viewed from above - as it would be seen by a predatory hawk - the bright green bird is very hard to spot against the lush canopy below. But once back at the nest entrance against the tree trunk, he becomes highly conspicuous to his partner and other male birds.

More info: heinsohn@cres.anu.edu.au

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