My first thought also, was that it “looks like” by the photo, that the feathers were taken out by another bird, or rubbed off, or something like that, rather than a medical reason. If you identify a reason such as that, separate the budgie from the reason for the bare patch, and you should see it resolve within weeks. If it persists, a visit to an avian vet would be advised.
We have lots of great up to date information on all aspects of budgie care. Take a look over our Stickies in each forum section. You’ll also find answers to questions you may have.
Wishing you all the best with your budgie