I'm first very sorry for your loss, I wish I had seen your post earlier, though chances are not much could be done for the poor little guy anyway, except making him more comfortable with antiinflammatories and pain meds. I bred budgies for over 20 years and have encountered this several times in newborns, juveniles after they had weaned and gone to their new homes, and in a few adult breeders. The causes were different based on the age of the bird, but in all of the babies and very young juveniles I always paid for a necropsy because I was breeding these birds and would never knowingly breed a pair that produced babies with fatal, congenital anomalies. I always had the necropsy done along with specific genetic blood work and organ testing.
In all cases of baby budgies I had that developed flat feet, paralyzed feet and/or toes, were unable to perch and had no perch response or perch reflex (you test this by pressing on the bottom of their foot where all the toes come together; if their foot automatically curls when you press at this spot where the toe tendons meet, their perch reflex is fine, if they do not curl their toes they have no perch reflex), and they had these symptoms WITHOUT SWELLING, it was always a congenital kidney issue. If caught early enough it can be stopped from progressing but the nerve damage cannot be reversed and the bird will be permanently disabled. They would often die very young anyway, even with me feeding them a very special, specific diet and administering several medications daily for the rest of their lives. And only a few times did I catch it early, simply by being aware of it once it had happened prior, and by being paranoid about it. Oddly, over 21 years I had this happen several times, though probably less than 10, and none of the babies were from the same parents or from even the same family line distantly. My avian vet actually discussed this with an avian genetic specialist he knows at the University of Pennsylvania School of Veterinary Medicine (he did his veterinary degree there before doing his avian specialization elsewhere; I live in State College, PA and he practices here, about 3 and a half hours away from Philadelphia) around 2006-2007, and this kidney disease that effects the line of nerves that in birds runs through the kidneys/kidney area and then a branch supplies the feet and toes, is an issue that they are born with, but does not seem to be a result of genetics but rather something that simply develops before birth, due to certain factors just lining up. It's not something that is passed on from parent to baby.
Incidentally, because you mentioned it, it does very much resemble gout in its end result, and I had considered gout as a possibility as well until having blood work and x-rays done. The main visible difference that can help distinguish a nerve issue caused by kidney disease from gout is that 99% of the time the bird will have a lot of swelling in it's feet and toes with gout and will have very little to usually no swelling with the kidney disease. Also, if a bird has gout they are typically in a great deal of pain and they stop eating, stop playing or being active, fluff up, and appear very ill, where as with the kidney disease the bird simply displays the nerve damage in the feet and toes and the inability to perch and walk correctly, that's it.
Again I'm very sorry you lost your little guy, but I thought it might help you to have an explanation from someone who has been through it and done a great amount of research on the topic with professionals, and to know there wasn't much that could have been done for the little guy.
"Dance like nobody's watching..."
