We have four Buff Orpington hens, three of which look pretty much like this fine lady here. Though she is turning away from the camera, you can see her red comb and wattle pigment. She also sports a standard-issue rump, with a nice feathery tail.
Then there is the fourth Orpington. She has no tail to speak of, and her comb and wattles are pale. Wondering about these differences, I turned to the interweb. A google serach for the exact phrase "assless Orpington" returned no results. Then I found that chicken fanciers have a much less vulgar vocabulary than my own: the correct term for types of chickens deficient in the booty is "rumpless." But, it turns out, there is no such thing as a "rumpless Orpington."
In the end (so to speak), it seems most likely that this hen has been bullied, and the bullies have removed her tail feathers. Orpingtons are kind of gentle, and apparently may be bullied by other breeds. This paints the other hens in an unflattering light, in which I do not like to think of them. When the new chicken habitat is finished, this chicken--and the other Orpingtons--will live in a separate habitat for peaceable chickens.