I am so sorry for you and your son's loss of Megatron. You both must be devastated, and I would probably just keep an eye on Jedi and quarantine him for a few days just to be sure. At this point, however, I agree with Wiki. He must have died very suddenly, and although he was separated from Bumblebee, I don't think this caused it since they still saw each other. It must have been something internal, which you had no control over. Sometimes these things happen, and Megatron knew he was loved until the end. I hope Jedi and the others take this okay, although it may take him a little while. Megatron was a beautiful little budgie boy and he is happy flying high over the rainbow bridge!
Budgies impact us in so many ways, sometimes I wonder how I ever lived without Mallorn's chirps and I know that when one day, she has to go, I will also be devastated, and humbled that she gave so much to me through all the years.
Rest in peace, little Megatron, and may his memory live forever
Also, I know that you might want to replace him in a little while so Jedi doesn't feel so lonely, but I would wait for a few weeks for the flock to get used to the new situation.
I hope only the best for you and your son in this difficult time