I have some birds (2 breeds from very different breeders hatched 6 months apart) that as chicks seemd to be fine. As they developed I noticed they seemd to have a labored gate and would occasionally fall back on their "knees". They don't stand erect like their siblings and now have a great deal of difficulty walking or even standing. They almost look drunk as they are very unstable while standing and stagger and wobble while walking occasionally plopping down to keep from falling. They have a good appitite and are nearly as big as the rest of their siblings. No other signs or symptoms, no sneezing, nothing.... Any clues?????