The Great Illusionist

Sitting at the centre of his court, this Great bowerbird male is surrounded by his collection of stones, plastic, glass and children's toys
Image credit: (copyright) Tim Laman

A new perspective: research suggests the male alters the females perspective of nearby objects through careful manipulation of object placement. 
Image credit: (copyright) Tim Laman

Throughout the forests and grasslands of Australia and New Guinea, many beautiful structures take shape among the undergrowth. At first these floor-bound stick structures may look like the nest of a bird with little sense, but these structures have an entirely different purpose. These are bowers; the unique and elaborate courts of male bowerbirds. 

In most species of bowerbird, the male lacks distinct morphological features (e.g. bright colours) that could be used to attract a mate. Instead males have evolved a unique behaviour for building elaborate bowers and adorning them with an unusual and exotic choice of objects. Male bowerbirds can spend 9-10 months of the year working on their bowers in the hope of attracting female attention. When a female does appear the male works hard to impress her with his collection of objects, vocal repertoire and dance display. 

These spheres are entirely identical but it is 
easy for backgrounds to alter our visual 
perception of size.
(measure them if you don't believe me)
Image credit: 123opticalillusions.com

Now a new study has suggested that for one species at least, males have learnt to exploit their potential mates by altering the visual perspectives of their court. 

Great bowerbirds (Ptilonorhynchus nuchalis) build 'avenue' style bowers (see above images) from which the female observes the males display. Observations have discovered that male Great bowerbirds arrange their court objects in increasing size with distance from the bower. Kelley & Endler suggests that this positive gradient causes the female inside the bower to experience an altered perception of object size. 

They reported that object manipulation  increased the chance of mating, particularly for males who had a regular increase in object size length ways, but not widthways across their court.

According to these researchers the possible cause of this increased mating success was due to females attention being held for longer whilst the male displayed. They hypothesise that the positive gradient may make objects stand out more from the surrounding forest floor or make the male and his favourite objects appear larger when close to the bower. 

Borgia, Coyle and Keagy contended this view, pointing out that male success is based on a variety of traits such as vocal mimicry, object number and colour, as well as bower quality and male dance display. But watch out... not all these traits are used as sexual cues in all bowerbird species. Great bowerbirds do not use vocal mimicry and object number is not related to mating success. 

The different perspectives of a female Great bowerbird from inside the bower (left & right columns). Notice how the black focal object (right) appears larger with a positive slope (A, top right) than neutral or negative slopes. Gray walls are the side of the bower, grey circles are objects of different sizes. Centre column is a top view of the male court.
Image credit: Kelley & Endler, 2012a

An interesting study! It would be great to know if the gradient of the court altered the females perception of the males size and not just object size (big males may have access to better resources). On a final note, the use of the term "gesso" by Kelley and Endler to differentiate between certain bowerbird objects is not substantiated. There is no indication that some objects should be more important that others in a males display; though males will sometime select a favourite object to present to the female. 

Find Out More:

VIDEO: Great bowerbird wins a female
VIDEO: How to woo a female
The story of 'Donald' - MacGregor's bowerbird and his quest to attract a mate. Article from National Geographic

VIDEO: The Great Bowerbird - Male does his best to attract a female

Sources:

• BORGIA, G. COYLE, B. J. & KEAGY, J. 2012. Comment on "Illusions Promote Mating Success in Great Bowerbirds". Science, 337.
• ENDLER, J. A., MIELKE, P. W. & KELLEY, L. A. 2012. Response to Comment on "Illusions Promote Mating Success in Great Bowerbirds". Science, 337
• KELLEY, L. A. & ENDLER, J. A. 2012a. Illusions Promote Mating Success in Great Bowerbirds. Science, 335, 335-338.
• KELLEY, L. A. & ENDLER, J. A. 2012b. Male great bowerbirds create forced perspective illusions with consistently different individual quality. Proceedings of the National Academy of Sciences of the United States of America, 109, 20980-20985.