What is this cyst type thing on my chicken.

[Mossies]

Hi Everyone. I am wondering if anyone might know what this fluid filled sack is on the back end of my Isa Brown chicken. She is about 6 months old. Thankyou for your help. Maureen

 

[Weeg]

Wow! That is very large! I am not gone be much help, but ti looks VERY irritated, and concerning. @azygous @Wyorp Rock @Eggcessive @Overo Mare will be much more helpful. I am sorry about your hen!

 

[Mossies]

It does not seem to be causing her any pain and she is eating and behaving normally.

 

[Eggcessive]

What is the black material next to the growth? Is that the vent? Where exactly is the growth? Is it below the vent opening? I wonder if she has a hernia.