Tiny differences in plumage and song have split two nearly identical birds into different species

In Iberá National Park in Argentina, two wren-size, nearly identical species of bird live side by side, spending their days foraging for the same kinds of seeds and nesting in the same kinds of places. These species can breed together successfully but normally don’t—for seemingly trivial reasons, evolutionary biologists have just discovered. Differences in belly color and song appear to be enough to keep the birds from mating with one another, generation after generation.

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