The Great Bowerbird, Ptilonorhynchus nuchalis

Bowerbirds are perhaps the most intriguing artists of the bird world. Their beautiful constructions are built purely to impress females (they are not nests, as often mistaken to be). One bowerbird, the Great Bowerbird, creates a particularly fantastic bower: in addition to building a symmetrical avenue made of carefully placed twigs, he also rearranges the objects on the ground (known as the ‘court’) around the bower. These objects are not strewn haphazardly, but instead carefully placed according to a system. First grey and white objects (often stones, shells, bones or man-made objects like bits of plastic) are placed on the court by their size, such that smaller items are placed closer to the avenue of the bower and larger objects are placed further away. On top of these drabber ornaments, brightly coloured objects are placed in specific locations that only the bower-builder could tell you the importance of (when a researcher moves one of these carefully placed objects the bowerbird quickly returns it to its ‘correct’ position).

Female bowerbirds move from bower to bower, inspecting male bowerbirds’ creations and the males themselves. Females often stand within the avenue of the bower, from which they can peruse the court and the male (who will display to her from there, often throwing his more brightly coloured objects around). Interestingly, the gradient that the male creates through placing objects in size order may act as a visual illusion from this angle, so that the female sees particular objects (and the male) as larger than they are in reality. One thing we know for sure is that females prefer males who have created steeper size gradients through their arrangement of the grey and white objects.

A male great bowerbird maintaining his bower:

However, the story does not stop there. Males differ in the degree to which they order their objects by size (i.e. the steepness of the gradients they create). Why might this be? Are some males ‘better’ artists, perhaps having a better eye for size, and so are more adept at creating these size gradients? This would mean that females could judge the ‘artistic’ (perceptual and cognitive) abilities of a male by looking at the gradient of his mosaic creation.

A recent study carried out by Natalie Doerr and John Endler at Deakin University in Australia sought to find out the answer to this question. To do this, the researchers found nine bowers in one location, and switched the white and grey objects in the court with the same objects in nine other bowers found in a different location. The researchers did not meticulously place these ‘new’ objects in size order for the male having his bower manipulated, but instead dumped them unceremoniously in front of his bower.

Males returned to find their bower in disarray

When the male bowerbird returned to his bower and found his artwork in total disarray, he quickly got to work rearranging the objects and re-creating the size gradient. The researchers then measured the gradient he created after he had been working on it for four days, and then again at eight days. This allowed them to see whether the same males that created particularly steep gradients originally now were the males that re-created steep gradients when given different objects to work with. In order words, were particular males the ‘better artists’ no matter what they had to work with?

What the researchers found was surprising. The steepness of the gradient created with the grey and white objects was not specific to a particular male, instead it more closely represented the gradient of the bower from which it had been taken. Being given the ‘tools’ of another male, the new male had created a structure rather similar to what the first male had done.

A male bowerbird tidies up the avenue of his bower

It seems then, in this case, that male bowerbird artists are not defined by their perceptual or cognitive abilities (i.e. their ability to see or think about where objects should go), but instead by the objects with which they have to use. This was oddly specific to even the type of object: males could make steeper gradients when they had more bones to work with, and fewer shells and stones.

At first glance this discovery might seem to imply that the male’s abilities do not matter at all- if his creation is solely due to the objects available in his immediate environment. However, this is unlikely. It is possible that there are even more intricate details of the court that female bowerbirds pay attention to in addition to the gradient, such as the fact that glass objects seem to be placed closer to the bower while bones are generally placed further away. Furthermore, the court is but one aspect of the entire display created by the male, which includes both other decorations as well as his dance display for the female. As it has taken evolution millions of years to create this unusual bird and it takes males years to learn how to build the perfect bower, it’s not surprising that it will take us humans a bit more time to reveal all the secrets of these avian artists.

 

Reference

Doerr, N. R., & Endler, J. A. (2015). Illusions vary because of the types of decorations at bowers, not male skill at arranging them, in great bowerbirds. Animal Behaviour, 99, 73-82.

All photos and videos taken by Natalie Doerr

Video of a male bowerbird displaying to a female before mating with her