Birds may decorate nests with artificial objects and feathers to deter magpies and ravens from taking their eggs.
Many birds have been observed placing human-made objects in their nests, such as plastic, wires or even anti-bird spikes from buildings. Often, these things seem to have no role as insulation or structural support, and may make nests more conspicuous to predators.
at Nord University and at the University of Oslo, both in Norway, suggest the use of unusual materials might help in defending nests from corvids such as magpies and ravens. Both birds are notorious egg thieves, but studies suggest they have an aversion to unfamiliar objects.
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鈥淲hy do birds do something that could make it easier for nest predators to find them?鈥� says Husby. 鈥淲e imagined nest decoration should have a positive effect and one reason could be that corvids were sceptical of novel objects and hesitated to rob such nests.鈥�
They also proposed that birds place large feathers on their nests to scare eggs thieves into believing a bird has been killed there.
To test these ideas, Husby and Slagsvold used three types of artificial nest. One simply contained quail eggs; another had eggs and a shiny metal spoon and the last contained eggs and large feathers. They conducted 78 trials in a forest to measure the responses of Eurasian magpies (Pica pica) and 60 trials at a landfill site to study common ravens (Corvus corax).
On average, magpies waited 96 hours before taking eggs from standard nests, 149 hours before stealing eggs from nests with a spoon and 152 hours before approaching nests with feathers. Ravens showed a similar pattern of behaviour, waiting 28, 34 and 43 hours respectively.
This hesitation could be crucial, giving birds more time to defend their nests and protect their eggs.
鈥淭his work has insightful connections to the growing body of literature on how some species of wildlife adapt to human structures,鈥� says at the University of New England in Maine.
Royal Society Open Science
