Visual effects created by bower birds
The great bower bird, Ptilonorhynchus nuchalis, signals his desire to potential mates with a display matched by few other birds. Eucalyptus forest in northern Queensland are the typical habitats for the species. The quality of his performance and the stimulation it gives to females are obvious, but there are multiple facets to this magnificent display.
The bower used for the "seduction" is designed to use the female's field of view and use illusion to create a false perspective! The courtship display uses coloured objects that flash to increase the effectiveness of his signalling. Other bower bird species use differing display techniques. The number of coloured decorations differ, the form of the display avenue changes and the "paint." Vocal mimicry is also used and movement can play a part too.
It's amazing that the great bowerbird male constructs a central avenue with a court on each side. The 1 metre avenue is furnished with parallel densely-thatched stick walls, while the courts are cleared, then covered with uncoloured stones, shells and bones. The female's very central position forces her to view the court in a special way to see the attractions. The male stands beside the avenue entrance where he can pick up, display and toss the coloured objects he has collected there. Her field of view (FOV) is restricted in a very special way.
The stick walls seem very significant to the female, taking up around 84-91% of her FOV, depending on whether there is a closed roof or not. The degree of redness of these sticks is greater at eye level than lower down, so the eyes are exposed to reddish light for most of the time. The colour measurements that the researchers carried out indicate that this alters the females perception of purple, green and red colours, especially her enhanced impression of the males beautiful nuchal crest. Red fruits should not be used by the male as display objects as they will appear more grey against this background.
The interesting result of this investigation is that this concentration on red interior walls had no effect on mating success. Maybe it just holds the female's attention or a simple redness threshold is enough to mate. The authors go into great depth on other possibilities here!
The male's great presentation must make the signal so effective that she visits fewer bowers. His contrasting display objects are enhanced by the avenue, while the female seems to enjoy a variety of suitable coloured objects. As many changes of colour as possible were attempted, although it was difficult to prove this had any effect on number of copulations. The males placed most objects used out of sight again within 3 days. The male's head was obviously important, with his neck and the objects in his beak prominent in her FOV.
The entire "ceremony" exploits the lady's field of view (FOV) and the rapid, diverse flashing of the male crest and appropriate objects to her. The concentration in this research was on the dramatic effect on her light adaptation by the redness of the surroundings. Tremendous work, with more research on the redness to come. The fascination could lie in the comparison with other species and discovering how the components of the display have evolved. Good luck with that, John A. Endler, Julie Gaburro1 and Laura A. Kelley, of Deakin University, Victoria, and James Cook University, Townsville, Australia. They publish today in - Visual effects in great bowerbird sexual displays and their implications for signal design.
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