City birds dazzle females with ‘borrowed’ human items

Bowerbirds in an Australian city use a range of human items – from glass and plastic to banknotes and even a pair of handcuffs – to impress females, new research shows.

Male bowerbirds create an intricate tunnel of twigs called a bower, then gather colourful items to show to any females that visit.

The new study, by the University of Exeter, compared items collected by bowerbirds in Townsville City and a rural area, both in Queensland, Australia.

City birds gathered larger, more colourful decorations – mostly from human sources – than their more understated rural rivals.

“Bowers are built exclusively to attract a mate, and males choose decorations that contrast against their own plumage and the bower itself,” said Caitlin Evans, from the Centre for Ecology and Conservation at Exeter’s Penryn Campus in Cornwall.

“Once a female arrives and stands in the bower, males throw an object into their view and then display the plumage on the back of their head, then throw another object – and so on.

“Our findings show that bowerbirds in a city use a wide range of items scavenged from humans.

“Glass, plastic and wire were common choices, but we also found items including a pair of handcuffs, medicine jars at bowers near a hospital, and fluorescent mouth guards from a site near an Australian Rules football ground.”

In urban bowers, red decorations were more vivid and green decorations were duller than those found in rural areas.

The researchers examined bowers of 61 males from the perspective of a female bowerbird – using existing evidence on their vision, which is more sensitive to colour than human eyes.

Evans said: “Our study did not assess whether females favour the more striking items collected in urban areas, but the males’ enthusiasm for gathering these items suggests this is likely.”

Urban bowerbirds collected more items – about 90 on average, compared to 20 for rural males.

One urban bird gathered more than 300 items at his bower.

When human items are not present, male bowerbirds typically collect fruit, seeds, leaves and sticks.

In this study, the two most common decorations in urban areas were green glass and red wire, while the most common in rural areas were green leaves/seeds and green glass.

Evans explained: “Even in rural areas, birds find items made by humans. In this case, we think they raid the bins and garage of a farm – and also the bowers of other male bowerbirds.”

In a second phase of the study, researchers collected 20 items – 10 from an urban bower and 10 from a rural one.

They presented these to both urban and rural males, and both groups strongly favoured human-made items.

Dr Laura Kelley, also from the University of Exeter, said: “Our study demonstrates that availability of human items – often glass and plastic – is affecting the behaviour of bowerbirds.

“We don’t yet know whether this has any negative or positive impact on them, but it’s a reminder of how human activity is changing the natural world in unanticipated ways.”  

The study was partly funded by the Natural Environment Research Council GW4+ Doctoral Training Partnership (NERC GW4+).

The paper, published in the journal Royal Society Open Science, is entitled: “Urbanization alters courtship signals in male great bowerbirds.”