Here you can see Scrap, the chick in the middle of the three in the picture. He has none of the black and white head feathers of the other two which look very similar to an adult.
Now that we are looking at the great tit chicks more closely, we see that one of them seems to suffer from very retarded development both when compared to the others and in absolute terms. After all it is 18 days since they hatched so they should be ready to leave the nest very soon. This immature chick, that we have been calling Scrap, looks anything but ready to leave!