Hello! My name is ***** ***** I would like to answer your question.
The most likely diagnosis for the lump that you are feeling is a lipoma, which is a type of benign tumor mass of fat cells. This is very common in older budgies, and generally causes few symptoms other that what you see, the mass in the lower part of his body between his legs.
It is possible to confirm the diagnosis with some cytology, where some cells are removed with a needle and then looked at under the microscope. In the hands of a competent avian veterinarian, lipomas can potentially be removed if they aren't too involved around other organs in the body.
There are other potential causes of the mass, of course - a variety of other tumors, both benign and malignant, is possible. Cytology can often differentiate between them with a minimally invasive procedure and very little discomfort to Bob. X-rays are often helpful as well.
Excess fat and low vitamin E in the diet (such as if he's on an all-seed diet) are partially to blame for the development of lipomas. The body has to store excess fat and it is generally deposited in the liver and in other tissues such as the skin. On top of that, some birds such as budgies have a genetic predisposition towards forming this growths.
Sometimes the lipoma can be treated initially with changes in the diet to reduce fat intake, and increasing the bird’s daily exercise. This means getting Bob to eat less fatty seeds and on to a diet that includes fruit, vegetables and pellets. Increasing vitamin E and biotin in the diet may also help. Sometimes the lump size will shrink and the need for major surgery will be reduced or eliminated altogether.
If the lipoma gets too big, then you may begin to see problems, such as difficulty walking/perching if the legs are affected or difficulty passing droppings if his cloaca starts to become obstructed. Also, the center of a very large lipoma can sometimes die off and become necrotic which may lead to a bacterial infection that may affect his whole body.
I would recommend talking to a local avian veterinarian about doing cytology on the mass to verify what it is. If Bob is on a mostly seeds diet, try to get him to start eating veggies , fruit and pellets. Budgies can be very stubborn about this, but with patience and time they can often be converted to a better diet.
I hope that this help answer your question, I am running out of time. If you have further questions, please feel free to ask!
Thank you for your help, bob has stopped eating his fresh apple pieces lately, so I will start getting him to eat this again and other health fruit, as taking him to a vet will stress him out and possible cause more problems, so first I will try the healthy eating way first, thank you, Avril.