Feeding old food or food which is not sterile can cause Ecolli, and other harmful bacterias to form in a crop. Always feed food which is not stale. Do not lick the syringe, and do not feed out of anything that has been in your mouth. Your mouth contains some really gross bacteria. If you use a microwave to sterilize your water prior to use, you could overheat the water and then feed it to a baby bird without knowing how hot it is, causing a burn which goes from the inside of the crop to the outside of the crop.
Prevention is simple.
Don t overheat the water. Test it on yourself before you feed it to a baby bird. Crop Burn can be agonizing and can lead to death . It is a horrible way to go. Use a thermometer or just take the time to test the water before feeding and you shouldn t have to deal with this.