This is one of a series presented by the National Aviary, which works to inspire respect for nature through an appreciation of birds.

What swims like a platypus, flies like a bumblebee and acts like a clown? The answer to this riddle is the ruddy duck, what one ornithologist called “a curious little duck … in a class by itself.”

Indeed, the ruddy duck is in a class by itself, at least in North America, being the only member of its tribe, the Oxyurinae. Worldwide there are only a half dozen species of similar ducks classified together in the genus Oxyura. (The scientific names come from the Greek word oxus, meaning sharp, and oura, meaning tail.)

Known colloquially as “stiff-tail” ducks, these six similar waterfowl species are distributed over six of the world’s seven continents, being absent only from Antarctica. The ranges of the six Oxyura are almost completely nonoverlapping; however, ruddy ducks that escaped from private waterfowl collections in England in the 1950s have become an invasive species there, threatening the survival of their native Oxyura, the white-headed duck.

Ruddy ducks get most of their food from under water. They make shallow dives lasting 15-20 seconds and use their large tactile bills to strain tubers, seeds, small crustaceans and aquatic insect larvae from the mud, much like a platypus does.

Because of their large rear-set feet (an adaptation for diving), ruddy ducks cannot walk easily on land, similar to loons and grebes. They share another trait with loons and grebes — proportionately very small wings. For ruddy ducks, taking flight requires pattering along the surface of the water for a long distance. When they do finally lift off, the blur of their furiously flapping short wings, in combination with their small stocky body, is what led one ornithologist to liken flying ruddy ducks to bumblebees.

The ruddy duck is a small reddish-brown bird with a disproportionately thick neck, large black-capped head, round white cheeks, over-sized bright sky-blue bill, and a short black pointy tail that it often holds erect. The female has the same big bill and stiff tail, but she is not brightly colored.

To human eyes, the male ruddy duck’s courtship display for the female can look comical: He opens by gliding toward her with his head raised high and his tail cocked, proceeds with some rapid head-bobbing and vigorous bill-slapping, throws in some bubbling-making and bill-dipping, and ends with an audible burp!

The male ruddy may help his mate with nest site selection, and later with the care of their young, but the female alone incubates her eggs, which, in proportion to her size, are the largest eggs of any waterfowl species. Just one more way this curious little duck is in a class by itself.

You can often find ruddy ducks on local lakes and rivers during the height of their migration in late November, and again in mid-March, but you can always see the “clown of the marsh” when you visit the Wetlands room at the National Aviary.

First Published: August 19, 2015, 4:00 a.m.