There are two races of Raja Shelduck recognised:
- Black-backed Radjah radjah radjah of New Guinea
- Red-backed R. r. rufitergum of northern Australia
The smallest of the shelducks, the Raja Shelduck is the only one to be found almost exclusively in the Tropics. In northern Australia it is often called the Burdekin Duck. Chiefly a coastal dweller, these ducks favour extensive areas of shallow, brackish water where they feed by dabbling, or grazing on nearby pasture. They are most active at night, roosting by day in trees or mangroves.
With their attractive plumage these ducks have long been popular in captivity, where they have the reputation of being by far the most gentle of all the shelducks. In the wild the drakes can, however, be exceedingly pugnacious, especially when the rainy season starts, as this marks the onset of the breeding season.
Most of the Raja Shelducks in collections are from the nominate New Guinea race, while rufitergum birds have been rarely kept outside Australia. Once established in a collection, these interesting ducks will breed quite freely. Despite being poor to eat, over-hunting has reduced populations considerably in Australia, while in New Guinea it has become both uncommon and local.
The Raja Shelduck likes to lay her clutch of 6-12 white eggs in a tree, invariably close to water. She incubates them without any help from her mate — incubation is typically 30 days.