Episode 7 – Why do bowerbirds hoard and steal blue stuff?

Male Satin Bowerbird (Ptilinorhynchus violaceus) tending to his bower.

Aleena and Janine respond to a listener request for an episode about their own scientific research. They’re happy to oblige! They kick off with Janine’s past research into satin bowerbirds and explain just why males collect, hoard, and steal so much random blue stuff! Meanwhile, Aleena discusses her own peculiar past habit of collecting local bus tickets, and Janine is trying to do a self-portrait – but not the standard kind (the existentialist kind, of course!).

What are bowerbirds?

There are 20 species of bowerbirds found in Australia and New Guinea. Males from almost all bowerbird species build elaborate structures known as bowers – this is where their courtship and mating happens. Bowers are not used as nests; females build separate nests in trees and raise their young without any help from males. Janine’s past research focussed on the satin bowerbird (Ptilinorhychus violaceus) which is found in many rainforests in Queensland and New South Wales. Satin bowerbird males are satin blue in colour and build their bowers on the ground using sticks, then decorate them with mostly blue items, plus some yellow and white items. Females, on the other hand, are a brownish-green.

Why do they collect blue stuff?

Females often perch in nearby trees and check out a male’s bower display. If a female is suitably impressed by a male’s bower and decoration haul, she may just choose to enter the bower avenue made of sticks (the bachelor pad). This will trigger the male’s full courtship dance (which is quite hilarious). If the female is digging the courstship, she will stick around and let the love-making proceed.

So collecting all the blue stuff has therefore been driven by sexual selection, with strong evolutionary pressure due to female choice. Females only mate with males whose bowers are sufficiently impressive, based on their own preferences (we think that because blue items are quite rare in nature, this is a good proxy for a male’s quality). There’s also evolutionary pressure for males to compete with each other – males steal decorations from one another and trash each others’ bowers so that their own display appears comparatively better to the discerning females. Check out this video from the Borgia Lab of a male absolutely going to town to destroy a rival’s bower, and watch all the way to the end when you see him stuff his beak with stolen decorations!

What did Janine’s research tell us?

Janine studied stealing of bower decorations. She found that the items most commonly stolen – crimson rosella tail feathers and blue plastic bottletops – reflected higher amounts of UV light than bower items never stolen. This suggests that UV-signalling is also a component of a male’s display to females.

Janine also found that males that stole a lot were also likely to be stolen from a lot. This suggests some level of tit-for-tat retaliatory behaviour. And the males that stole the most also ‘painted’ their bowers more than other males. We already know that this painting behaviourseems to be related to mating success, so it looks like stealing decorations is another key strategy for impressing the ladies.

And THEN…. Janine did an experimental manipulation that placed all males on a ‘level playing field’ in terms of their numbers of decorations. Interestingly, after 3 weeks the numbers of decorations shifted back to being the same relative proportions as before: the better quality males restored their better collections, while the lower quality males couldn’t quite protect their newfound haul so well… This showed that bower decoration number does seem to be an honest signal: there really is something inherently different about males with lots of decorations, and trying to give less successful males a ‘leg-up’ really doesn’t work!

Wojcieszek, J. M., Nicholls, J. A., & Goldizen, A. W. (2007). Stealing behavior and the maintenance of a visual display in the satin bowerbird. Behavioral Ecology, 18(4), 689-695.

Wojcieszek, J. M., Nicholls, J. A., Marshall, N. J., & Goldizen, A. W. (2006). Theft of bower decorations among male Satin Bowerbirds (Ptilonorhynchus violaceus): why are some decorations more popular than others?. Emu-Austral Ornithology, 106(3), 175-180.

What’s the latest in our understanding of bowerbird evolution?

The satin bowerbirds are just one of the 20 species of bowerbirds. There is great diversity in the styles of bowers. Some of the other species build similar ‘avenue’ bowers to the satin bowerbird, but the colours of decorations differs by species. Then there are the bowerbirds that display leaves on simple cleared ‘courts’ all the way to the ‘maypole’ bowers – the most impressive of which, the vogelkop bowerbird, builds such an elaborate bower, it really needs to be seen to be believed.

We had thought that bower-building evolved once in a common ancestor and then diversified among the different species, but a 2020 comprehensive genetic analysis (250 gigabases of DNA data!) has shown that it is more likely that bower-building evolved two times independently. This is a very exciting finding for evolutionary biology nerds! It seems that the tropical rainforests, where these birds are found, have less predators than other habitats. So bowerbirds have more time on their… legs(?) to get building, which means these weird and wonderful behaviours are more likely to evolve and persist!

Ericson, P. G., Irestedt, M., Nylander, J. A., Christidis, L., Joseph, L., & Qu, Y. (2020). Parallel evolution of bower-building behavior in two groups of bowerbirds suggested by phylogenomics. Systematic biology, 69(5), 820-829.

What brought out our inner squares?

Aleena discloses a brief and inexplicable phase during her childhood in which she passionately collected local bus tickets and proudly displayed them in those folders usually reserved for baseball cards. This doesn’t have anything to do with bowerbirds. But she took the opportunity to find the current Guinness World Record holder for most number of bus tickets. So apparently it’s a thing.

Janine has been reading existentialist literature (who hasn’t?! hehe), and stumbled on this self-portrait by physicist and philosopher Ernst Mach from 1886. It is a self-portrait from the perspective of what Ernst sees out of his left eye, including bits of himself… Of course, Janine has been trying to replicate this, and it is much harder than it looks! Aleena is reminded of the ‘sausages or legs‘ conundrum but Janine is not so sure…!

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