lump under my parakeet's neck

by lulu
(North Brunswick, NJ)

My parakeet has been pulling out feathers under his chin. He usually does this and they grow back. However, I noticed a big lump under his neck on this exposed skin that has spots that look like a lot
of food (seed) he probably ate with the husk. Is this normal? I am concerned. Please advise.
Also, why does he pluck his feathers out so much that he always ends up with a bald patch somewhere? We have another parakeet in the same cage who does not moult or pluck its feathers.
What can stop him from plucking so much?.