The Red-backed Fairy-Wren, a tiny songbird living the Australian scrublands, is highly territorial and promiscuous. The male can’t be sure the eggs in his nest are his own. One way to help avoid this problem? The male may rough up a rival who approaches his territory. But research shows when Red-backed Fairy-Wren pairs sing a duet in order to deter rival males from intruding on their territory, those pairs had more of their own genetic offspring in their nests. For the male, aggression might not ensure paternity, but singing a duet could.

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