Some species of birds try to save energy by tricking others into incubating their eggs. But if the parasitic female is a related species, she may have an advantage. After studying the nests of Common Eiders, such as the one pictured here, researchers at the University of Gothenburg in Sweden determined that trickery among close relatives of the nest owners caused no aggression. But the attempts of unrelated females often resulted in conflict. 

More info and transcript at BirdNote.org. 

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