The days on which the slim boy’s cocker spaniel eyes shone brighter than ever, and his brown wavy hair seemed to have a movement of their own, they knew that he must have read a ghost tale and was hatching a plot to scare someone.
I had read something like this, and previously asked a question about "cocker spaniel eyes" , from which I've got some very helpful answers. But a new question was aroused then, as what Mr. PeterG said:
I thought that point which hadn't been noticed was quite interesting. After thinking over that, I thought I would see it this way:
"his brown wavy hair seemed to have a movement of its own" can be understood in a way that his hair, as a whole mass, goes in a same direction of its own. But I have a different understanding that the author actually uses "their" to mean each strand of his hair goes in its own direction, that is to say, the boy has rough or Afro hair.
"his brown wavy hair" is deemed to be a mass noun with a singular form, whereas "have a movement of their own" has to lay stress on each individual hair's own movement, and in this case, the author has no way to avoid making "hair" and "their" contradict each other according to strict singular/plural rules. In a word, I don't think it's a mistake. Here, logic should be given greater priority over some rigid grammar rules.
It seems that Mr. PeterG doesn't agree to regarding it as a right sentence. I am not a native speaker, I am somewhat ready to accept his argument, but I still need more evidence to be convinced.
Is it grammatically wrong to use ”their“ in this case to refer to hair？