National Geographic

Male bowerbirds use forced perspective architecture to get more sex

This is an updated version of an old piece, edited to include new information. Science progresses by adding new data to an ever-growing picture. Why should science writing be different?

Right from its entrance, Disneyland is designed to cast an illusion upon its visitors. The first area –  Main Street – seems to stretch for miles towards the towering castle in the distance. All of this relies on visual trickery. The castle’s upper bricks and the upper levels of Main Street’s buildings are much smaller than their ground-level counterparts, making everything seem taller. The buildings are also angled towards the castle, which makes Main Street seem longer, building the anticipation of guests.

These techniques are examples of forced perspective, a trick of the eye that makes objects seem bigger or smaller, further or closer than they actually are. These illusions were used by classical architects to make their buildings seem grander, by filmmakers to make humans look like hobbits, and by photographers to create amusing shots. But humans aren’t the only animals to use forced perspective. In the forests of Australia, the male great bowerbird uses the same effect to woo his mate.

Bowerbirds are relatives of crows and jays that live in Australian and New Guinea. There are 20 or so species. In most of them, the male attracts mates by building an intricate structure called a bower, which he decorates with specially chosen objects. Some species favour blue trinkets; others collect a  mishmash of flowers, fruits, insect shells and more. Surrounded by these knick-knacks, the artistic male performs an elaborate display. The females judge him on his skill as a performer, builder and decorator.

The great bowerbird’s taste for interior design seems quite Spartan compared to his relatives. He creates an avenue of sticks, around 60 centimetres long, leading up to a courtyard. The courts are decorated with gesso – a collection of gray and white objects including shells, bones and pebbles.

The male performs in this messy courtyard. He struts. He sings. He tosses brightly coloured objects about. All the while, the female watches from the lined avenue. Her point of view is fixed and narrow, and according to John Endler, the male knows how to exploit that.

In 2010, Endler showed that the males place the largest objects towards the rear of the courtyard and the smallest objects in the front near the avenue. This creates forced perspective. From the female’s point of view, it looks like the bigger objects, which are further away, are the same size as the smaller objects, which are close by. If bowerbird vision is anything like humans, the courtyard as a whole looks smaller to a watching female. It’s the opposite effect to the one that Disney visitors experience.

By analysing 19 different bowers across Queensland, Endler showed that the arrangement of objects in the courtyards were far from random. When he messed up the careful gradients by reversing the large and small trinkets, the males were quick to put things right. Within three days, the illusions had been restored.

Now, Endler has shown why the bowerbirds are so fussy. With his colleague Laura Kelley, he has found that the males who create the strongest illusions get the most mates. From the female’s point of view, the courts with the strongest forced perspective and the most even patterns earned their owners the most sex. “To my knowledge no other animals make constructions which produce perspective,” says Endler.

Perspective has been a familiar element of Western art since the Renaissance, and Endler describes the great bowerbird’s courtyards as “bowerbird art”. Others would agree. In this clip David Attenborough compares the work of a Macgregor’s bowerbird to a sculpture by British artist Andy Goldsworthy and asks why one might be considered art and the other not. Defining art is a tricky business, but Endler thinks of it as one individual creating a visual pattern in the outside world to influence the behaviour of others. “Influencing behaviour can range from attraction to and voluntary viewing of the art by others to viewers mating with the artist, which is what bowerbirds do,” he says.

The bowers might be art, but are they actually illusions? It’s not clear. Despite Kelley and Endler’s new study, we’re no closer to knowing exactly why the bower gradients work. Perhaps more regular pattern on the court, as seen from the avenue, could make the male more conspicuous or easier to see. The same applies to the object that he waves about – perhaps it stands out more against such a regular background, or seems bigger.

Endler notes that from the female’s perspective, the male’s display items are often slightly larger than the ones in the gesso. This could trigger the ‘Ebbinghaus illusion’, where objects seem bigger if they’re next to smaller ones than next to larger ones.

But there are explanations that could account for the male’s habits without having to invoke any illusions. Perhaps it’s simply that the female likes a more uniform texture. Maybe she recognises that males who can produce regular patterns might be mentally sharper, better at stealing the right objects from other bowerbirds, better at resisting such acts of thievery, or better at choosing building sites with lots of varied objects to choose from.

In a related editorial, Barton Anderson from the University of Sydney writes that we still don’t know if the bowerbirds are actually crafting illusions for their mates. He says, “Kelley and Endler’s data suggest that male bowerbirds appear to consider the viewpoint of their potential mates when constructing their bower courtyards, and the ones who do this best are rewarded with a higher rate of mating success. Just what matters, and why it matters, remain open and intriguing questions.”

There’s plenty of time to answer them. After all, the bowerbird’s behaviour has only just been discovered. Back in 2010, Endler says that he still needs to do the “critical experiments” to see how much brainpower the birds need to pull off their illusions. That’s still the case in 2012.

The most extreme explanation is that they have a sense of perspective (insert joke about humans here) and know that they should put small objects near the avenue and bigger objects further away. But Endler says, “The very simplest hypothesis is that the birds make the gradients by trial and error.” They spend around 80% of their time at the bower on moving objects within the courts, checking the view from the avenue, and moving things again.

“They could just be doing that until the view of the court from inside the avenue looks ‘good’,” says Endler.  “A slightly more complex behaviour might be needed if they had an inherited or learned decision rule which made them put smaller objects closer to the avenue entrance and increasingly larger objects further away.”  Neither technique would be unexpected, given that bowerbirds are closely related to some of the smartest of all birds – crows, ravens, jays and their kin.

References: Kelley & Endler. 2012. Illusions Promote Mating Success in Great Bowerbirds. Science http://dx.doi.org/10.1126/science.1212443

Endler, Endler & Doerr. 2010. Great Bowerbirds Create Theaters with Forced Perspective When Seen by Their Audience. Current Biology http://dx.doi.org/10.1016/j.cub.2010.08.033

Credits: Photos by Endler; videos by Keller

There are 5 Comments. Add Yours.

  1. Dan
    January 19, 2012

    Links to video lead to a “This video is private.” message. Rats.

  2. grandma shirley
    January 20, 2012

    Even if we never know why the birds do what they do, it’s still fun to speculate. I love science!

  3. IW
    January 23, 2012

    There seems to be an implicit assumption here that the birds view these things the same way we humans do. What evidence is there, that this is the case? We have binocular vision, the birds do not; has this been taken into account?

  4. Wang-Lo
    January 25, 2012

    Looking through the bower, and trying to see like a bird, my first impression was that I was looking at a bird’s nest made of masonry. Surely this would be attractive to an oviparous househunter. It also looks a bit like a stone wall, which would also demonstrate amazing prowess in construction.

    After all, these creatures cannot shop at Home Depot, and the racket made by power tools would certainly attract predators.

    -Wang-Lo.

  5. George
    February 5, 2012

    Andersson point out in his Nature article that the forced perspective workks opposite to what Kelly and Endler says …the way its laid out it makes the male look smaller rather than bigger. They also don’t use more than half of their data and base their conclusions on 8 bowers which seems strange for a Science article. This may all be a bunch of bunk.

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