Hi there! Whenever a young chick has mobility issues, 2 things come to mind. One is a slipped tendon, which usually accompanies a very swollen hock joint, and they usually have a consistent difficult time walking. The other, which I believe is the case with your chick, would be Marek's Disease. Even if vaccinated some can be afflicted. It usually starts with odd walking and progresses to not being able to walk at all. This especially makes sense if she is much smaller than the others as well-it could mean the virus is causing problems with how she is absorbing her food.
However, since I cannot palpate her leg myself, I would advise a veterinarian taking a feel of it if you have one in your area that will do so. If not, I would suggest separating her so she doesn't have to compete for food or water, or move around too much, and see how she does.
Unfortunately, if it is Marek's disease, it can be contagious, though not all birds exposed will become afflicted. This is a very common virus in almost all backyard flocks, even with strict vaccination protocols.
Ah yes, then it makes the likelihood of Mareks being the cause quite a bit higher.
That is the normal but it can afflict younger ones. And there can be many other signs than just the split leg appearance.
Other causes could be trauma, though that shouldn't cause a smaller size unless she is being pushed away from the food. It could be a tendon issue from development, again wouldn't cause small size unless being bullied. Prognosis for slipped tendons are usually not good even with a brace, but sometimes worth a try if that is what it is. Do you have a veterinarian in your area that will see chickens?
Yes it can be difficult to justify that much for a small chick, and they might not be able to do anything anyway. I would try separating her so there is no competition for nutrition. I would also recommend using Rooster Booster Poultry Cell as a vitamin water additive as if there is a deficiency this may help.
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