Separation of two almost identical bird species by small differences in plumage and song

In Iberá National Park in Argentina, two almost identical bird species the size of a kingfisher live side by side, spending their days looking for the same types of seeds and nesting in the same types of places. These species can reproduce together successfully, but they usually don't, for seemingly trivial reasons, evolutionary biologists have just discovered. Differences in belly color and song seem to be enough to prevent birds from mating with each other, generation after generation.

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