Looks like its toe tips are going to rot off.. Usually I'd say something has been restricting the blod flow, causing the tissue to die, but as it's happening only at the tips of the toes and on all the toes, and there doesn't seem to be anything tangled around the toes, I'm not quite sure about that. It kind of looks like frost damage - the extremes of the body are affected first, because that's where the blood has most trouble keeping you warm. But I doubt you've exposed it to frost ^^ So, my guesses:
1) Button chick feet could be very sensitive to cold and going cold during the first few days of its life could have caused so little blood to get to the feet for so long that this process started.
2) Maybe it's the opposite - could the heating pad be too warm? But I guess you'd expect damage at the bottom of the feet then, and probably not looking like this very typical restricted blodflow damage (gangrene)..
3) Does motherconure ever preen his toes? She could have bitten a little too hard, causing the blood vessels to rupture or something..
4) Something genetically wrong with the chick, for instance causing the skin on the toes to be too tight, not allowing blood out there as it grows.
5) Something (poop? Hay? Hair?) has been wrapped around his toes for too long at one point.
6) Oyster shell only puts the organs under pressure on a small chick that doesn't need it, so I would stop using it. I don't know if it can cause anything like this though..
If the cause of the problem has been removed (but I guess it's hard to tell if it has), I wouldn't worry too much about it. I think he is going to loose the tips of his toes no matter what you do. It might get a nasty, exposed bone-like look, but I don't think it'll hurt the chick and it'll most likely fall of on its own, though it might take a while.
Good luck to both of you
