Patricia Brennan strolls through an outdoor aviary with a net in hand. She scoops up a duck, turns it onto its back, and applies pressure to its belly. "If you know exactly where to press, you can pop the penis out," she explains. "They're quite cooperative. The males get used to being handled."

The biologist didn't plan to spend more than a decade coaxing ducks to reveal their genitals to her. It wasn't until the end of graduate school that she realized birds could have penises at all. And with good reason: the vast majority of birds – 97 percent – don't. Ducks are among the small minority that does.

Owing to an odd quirk in their physiology, ducks develop