Instructions
Grasp the duckling, gently, cupping it between your hands. Turn the duckling upside down, so the duckling's tail is pointing straight up.
Fold the duckling's tail backwards with an index finger to reveal the vent on the duckling's underside. Place light pressure on both sides of the duckling's vent with your thumbs while moving your thumbs outward, to the sides, gently pulling the skin away from the vent. Do not apply any pressure to the abdomen, or you risk injuring the ducking.
Look inside the duckling's vent as you are spreading the skin. If the ducking is male, the penis shaft will be visible in the vent. A small circular knob indicates the duckling is female. If neither is visible, you have not preformed the procedure correctly.
How to Tell a Male Duck From a Female Duck When They're Babies
As the sex organs of most birds are not exposed to view, determining their sex can be challenging. While far from being a 100 percent reliable indicator, male ducks are generally more brightly colored than females from birth though adulthood. The only way to definitively tell a male duck from a female duck when they are babies is by sexing the duckling by a process called "venting." The technique requires practice and patience and involves spreading the protective skin covering from around the cloaca while looking for a penis or lack thereof.