Genders

? A Bump where exactly?

Never ever heard of sexing chickens by bumps so going to go with no pending more information.

 

Chickens have no external genitalia. All birds, M & F have a single opening, the cloaca. It handles all functions; urinary, intestine excretory and reproductive.

 

The only bump I can think of is the tail. Is that what you mean? If it's not that then you need to post a photo of this bump you are talking about.

 

It is the tail. As your chicks grow you will see tail feathers sprouting from there. Look up chicken skeletons on Google and you will see that there are a few tail bones in that bump.

 

There is a bump at the end of the back right before the tail starts. It is an oil gland birds have to use while preening their feathers.
Both male and female birds have this.