Hi Bently, first " Welcome to the group" second, thats not the rump bump I am talking about. Some of my chickens have a fluffier butt under their feathers, it causes their butt to look, for lack of a better term, like J-lo's or like they have a big butt. It also has to do with how they hold their wings.
I was holding birds and looking at behinds yesterday and noticed that some of my hens and roos have alot of underfluff that extends even up their belly, while others have almost none. The fluffier the bird the more messy of a behind they will get also.
Look at this pullet for instance. She has a definate bump on her rump ( poofy backside ) not from a partial tailbone, but from fluff. You can see her fluff stick out behind her wings. All that fluff causes her butt to look more rounded.
Here is another fluffy butted bird. Not a bird with a partial tailbone. It is very easy to tell the difference when they are day old chicks.
The above bird is the bird on the left in this pic.
I trimmed this guy up yesterday because he was almost all fluff. It was pretty ridiculous. It looks like a chicken exploded around my chair outside. There is fluff everywhere.
Look at the area below his wings, that is all just fluff.
This guy here has the perfect amount of fluff, and his backside is always neat and clean. His attitude is that he would rather die than have me clean his behind. LOL!
All of the above pictured birds are fully rumpless. Some have more fluff than others.
Lanae