Saturday, June 19, 2021

Why do male bowerbirds decorate their nests?

To woo females, the males of 17 of the 20 known species of bowerbirds build structures — often resembling an arbor, or bower, with an artfully decorated platform. 

Scientists are drawn to bowerbirds because they clearly show the power of sexual selection, the evolutionary force that Charles Darwin defined to explain conspicuous male traits such as song, bright colours, and horns. In most animal species, Darwin noted, females do the choosing — basing their decision on the ornamentation and ostentation males use to attract them. Because most bowerbirds are polygynous, meaning one male is the mate of more than one female, and these males build decorative bowers, they make excellent species for testing this idea. Males don’t help the female build a nest, incubate the eggs, or raise the chicks — all they give her are their genes. The females are thus very choosy about which male they pick.

Evolutionary biologist Jared Diamond has called bowerbirds ‘the most intriguingly human of birds.’ These are birds that can build a hut that looks like a doll’s house; they arrange flowers, leaves, and mushrooms in an artistic manner; some can sing simultaneously both the male and female parts of another species’ duet, and others easily imitate the raucous laugh of a kookaburra or the roar of a chain saw. Plus, they all dance. And bowerbirds were found to kill beetles solely for the purpose of decorating. Humans are the only other species known to use animals in this way.

Given all these talents, some researchers have attributed an aesthetic sense and the glimmerings of culture to bowerbirds, traits rarely suggested as found in any species than humans.

All the bowerbird builders are picky about what they collect and fussy about their arrangements. 

In Australia, satin bowerbird displays blue parrot feathers, white snail shells, and yellow and purple blossoms. For sheer obsessive collecting, a few males, amass thousands of white and gray pebbles, snail shells and sheep vertebrae, piles of green and purple glass, rifle shell casings, colourful plastic strips, wire, bottle caps, tinfoil, mirrors — in short, almost anything bright and shiny, even CDs. 

These things please females and the males also use them in competing with one another. They fight, steal each other’s ornaments, and shred each other’s bowers. Simply, they fight over their stuff!

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