The short answer for a Pekin duckling
A young Pekin duck is sexed by its voice: the female grows into a loud, carrying, unmistakable quack, and the male never does — his adult voice stays a hoarse, low rasp that you have to be close to hear. Pekins are an all-white breed with identical plumage on both sexes, so feathers offer almost nothing until a drake's curled tail feather arrives. Your ears, used at the right age and one duckling at a time, settle the question weeks before your eyes can.
Why voice works so well in Pekins
A Pekin duckling of either sex peeps for its first weeks, so early listening tells you nothing. The change comes as the young duck's voice box matures: the female's develops the structure that produces that classic resonant quack, while the male's produces only a breathy croak. Size is a genuine trap here — Pekins grow fast and unevenly, and a big, bold duckling is just as often a female as a drake.
The listening test, done properly
Sexing a young Pekin by voice works when each duckling is tested alone. Carry one duckling a short distance from the group; separated from its flock-mates, it calls, and the call is either a full quack or a rasp. Test from about six weeks onward, and repeat a week later for any bird whose voice still sounds in-between, because voices change on slightly different schedules and a late female can still sound fuzzy at the first attempt. A duckling that stays silent tells you nothing — set it down, let it settle, and try again another day rather than squeezing or startling it into sound. Two rounds of solo listening, a week apart, correctly sort nearly every Pekin in a typical backyard batch.
The drake curl and other confirming signs
A young Pekin drake confirms himself with the drake curl: one or two tail feathers that arc up and curl forward over his back, usually appearing somewhere around eight to twelve weeks. A female's tail stays flat and straight, so the curl is a one-way sign — present means male, absent at a young age means only 'not yet proven.' Drakes also tend to carry their bodies a touch lower and longer, and as they mature they go quieter while the females get louder and more conversational. Vent sexing exists for day-old ducklings, but it is a skilled technique that can injure a duckling done badly, so it belongs to experienced hatchery hands rather than a first-time keeper's kitchen table.
Mistakes that mislead Pekin keepers
The classic error with a young Pekin is sexing by behaviour at the water bowl — the duckling that charges in first, or mounts others in play, gets labelled male, yet both sexes behave boldly in a fast-growing Pekin group. Size misleads too, since Pekin ducklings put on weight at different rates depending on hatch order and appetite, and the heaviest bird in the pen is no evidence of anything. Write down each duckling's solo-voice result rather than the group's general impression, and let the curl feather revise the list, not your hopes about it.
Keeping the results straight as they grow
A Pekin duckling's sex matters for egg expectations, for drake-to-duck ratios as the flock matures, and for separating birds before breeding season behaviour begins. Mark each duckling with a loose coloured leg band the day its voice test is done, so the bird you tested is the bird you think it is when the flock is milling around the feeder. A duckling of either sex that goes off its feed, sits hunched away from the group, or breathes with its bill open needs a veterinarian promptly rather than another round of sexing tests. Logging each Pekin's voice checks and curl sightings through KinsLe's Care Teams keeps every helper's observations attached to the right banded bird, and the timestamped entries show at a glance which ducklings still owe you a second listen.