Update: The vet said it's possible that Trio's cere is bruised, which wouldn't be surprising, given he spends a lot of time outside the cage and occasionally has his flight path disrupted by one of the other birds.
If the cere discoloration doesn't resolve in a month or two, the vet will do an X-ray with barium contrast and, if there is a testicular tumor, put him on Lupron, which she said has been shown anecdotally to reduce the size of the testicles.
She said that given Trio's age (less than three years old) and the fact that there are no other physical or behavioral changes, there's not an immediate need to do the X-ray. Obviously, I'm going to be keeping a very close eye on him. Fingers crossed!
A few of mine get that... Or else I am not seeing something wrong properly!? In my case the avian vet told me it can happen sometimes depending on hormonal fluctuations. Mine had it go away. Some then back and back away....
Yep... In my case I recall the last one to show that had it slowly reduce in size and depth of the different Part until it vanished from early March to mid April (from Summer to Fall where I live) and also shed snd grew a few feathers and was changing in behavior (interest in his partner gender)