Why Is Budgie Rubbing Vent Area?

I have a budgie who is eating normally and looks well. But recently has started to rub his vent on the perches in his cage. I have looked and his vent and it looks ok, with no stained feathers, although it looks bluish rather than the usually pink color. Also I noticed a small yellow patch under his skin the size of a seed. That does appear to move when pressed. Is this likely to be the cause an infected cyst that needs to got o the vets? Or will it go away like a spot?

It is a good idea to get your bird checked out by a vet, as there are a few different conditions that can cause irritation or trauma of the vent area. For example, cloacitis following on from cloacal stones can cause soiling around the vent and subsequent irritation. There are also different causes of lumps in the skin that can include Lipoma (fatty lumps), Papillomas (warty growths), or a bacterial infection.