In a study of nearly 500 wild birds from five common species in Australia, researchers found that sex reversal, when a bird’s genetic sex does not match its reproductive anatomy, is more common than scientists once believed. About 5 percent of the birds showed this mismatch, and most cases involved genetically female birds that had developed male reproductive organs. In one example, a genetically male kookaburra appeared to have laid an egg. Those findings suggest there may be environmental or biological factors or maybe even social factors in birds that we do not yet fully understand.

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