Had a baby hatch under hen yesterday. Came home and looks like the hen pulled on the umbilical cord and killed her. I have 7 more eggs under her. How can I prevent this on the next ones that might hatch? :0(
Chicks don't have "umbilical cords". They do absorb the yolk sack into their stomach, but sometimes hatch with the navel still open. If this happens a chick may even flail enough to have their intestines comes out. I had this happen once - it was not pretty. THe mom may also have pecked at the open navel and pulled the intestines out. Hopefully the others will not hatch with open navels.
Chicks DO have umbilical cords, its how they absorb nutrients during development and the yolk before hatching. And you are correct, it looks like a little black string.
If your broody is doing her business in a coop full of other chickens, someone else might have committed the foul (or fowl?) deed. If she's separated, then it was probably her :/ I don't see an issue with cutting it short to avoid another accident, as long as its already dried up. Good luck, sorry about your chick!
They don't have true "umbilical cords" in the same way that mammals do. They get the nourishment from the yolk from veins and arteries that develop from the chick into the yolk. Sometimes when the chick hatches the yolk has not been fully absorbed yet and when it finishes absorbing it there can be a remnant of the attachment that will dry up and fall off. It is kind of like an umbilical cord, so I guess that is why it is often called such.
If the navel is not healed but still partially open, tugging on it could pull the yolk sack and even the intestines out, though sometimes a chick in its own flailing may pull on it itself.