What is this black spot or thin string attached to my baby chick's rear end?

It's your chick's umbilical cord, and if you notice, that black, scabby spot or string will be below your check's vent or "pooper." Be SURE not to confuse this with pasting, which occurs on the vent, not below it. Usually the umbilical cord falls off immediately during hatching, but sometimes it can hang around for a few days (or it falls off but the belly button takes a while to heal, leaving a scabby spot). This can happen whether you hatch at home, or if you order from My Pet Chicken or another hatchery.

Please don't try to remove it, as that can cause excessive bleeding for your new baby, and you do NOT want the umbilicus to become infected. It'll eventually fall off on its own, and the spot will heal, just like it does for a human infant.

If the other chicks seem to be picking at it, you may have to separate this chick until it does fall off. You can also use a little iodine on the umbilicus to help speed the process of drying and to ensure there is no infection. Iodine is the best thing you can get at home to use for this---don't use peroxide or alcohol. Use iodine. Just a dab once a day for two or three days should be plenty.