http://www.nytimes.com/2007/05/01/science/01duck.html
The anatomy of ducks is especially bizarre considering that 97 percent of all bird species have no phallus at all. Most male birds just deliver their sperm through an opening. Dr. Brennan is investigating how this sexual wonder of the world came to be.
Part of the answer, she has discovered, has gone overlooked for decades. Male ducks may have such extreme genitals because the females do too. The birds are locked in an evolutionary struggle for reproductive success......
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