Male chickens have tiny nubs for reproduction, while ducks and some other birds are much better endowed. It raises a question perhaps as perplexing as "Why did the chicken cross the road?"
A National Geographic story published in 2013 but still bouncing virally around social networks offers the answer uncovered by bird penis researchers.
A protein known to scientists as Bmp4 nips a chicken's developing penis in the bud, so to speak, in its first week of development, while a duck's keeps growing into a corkscrew appendage useful for fowl reproduction, according to the research. The protein stays away from the developing duck's reproductive organ. It's the same protein that shapes your body parts by causing a "mass suicide" of cells in the embryo. Without it, you would have not have fingers.
One theory about the de-evolution of chicken penises says that hens gradually chose roosters with smaller penises so they could have more control over reproduction. As it stands now, the females are not as vulnerable to unwanted mating because it is tougher for the male to transfer sperm with a nub.
If you want to know all about bird sex, read Ed Yong's "How Chickens Lost Their Penises (And Ducks Kept Theirs)."
If you just want to see what a duck is hiding under those feathers, then take a few seconds to watch this video showing "the explosive eversion of a duck penis in air titled "The Explosive Sex Life of a Duck."
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