What is my chicken imprinting on?

I have a whole group of chickens (about 8 of them), but they don't have a mother. When they were first hatched, I didn't really get to see them at all until about their third day after hatching. So, then to what did these chickens imprint onto? One observation I have made is that, if you put one chicken outside of the coop where the other chickens are, he/she will just run right back over to the coop and tap on the gl and stare at the other chickens. Does this mean that the chickens have imprinted on other chicks?