What do i do if i have white string in my feces?
It can be undigested material or a sign of threadworms. You should definitely see a gastroentenologist. He/she will most likely suggest stool testing. (This is not something you should ignore.)
Birds do not urinate as people do. Their urine is excreted with their feces. Their feces is actually brownish-black in color and the white you see mixed in with it is the "urine", which is actually very concentrated urea, not urine itself. Also, usually the bird feces can be white with a black spot or dot, not the other way around.