Is the feed medicated? How do you know the silkie has sour crop? If she does have sour crop stop giving her anything to eat for 24 hours and just give her water with quality apple cider vinegar to drink. Wait until her crop is completely empty (you can feel if the crop is empty) then only feed her plain yogurt and little scrambled eggs, little apple sauce etc... NO feed for at least 3 days than only small amounts with the yogurt etcc.... Stop the feed again if she doesn't have an empty crop by morning each day. Go back to the diet and try the feed again after 2 days. My vet said use a 10 day course of tylan 50 by injection for my other sick birds that had coughing, respiratory issues etc... My birds were not eating their feed, I was giving them to many scraps and they ended up with a pretty serious vitamin e deficiency. After treating the illnesses caused by their bad diet and adding the vitamins they needed to get back on track my birds were 100 percent better, but it took awhile.