Learn English – Family or families

"Are they?" is the standard question form. In a question, we normally begin the sentence with an interrogative word like "where" or "how", or "is" or "are" or "do" or "does".

"They are?" is an example of a declarative statement turned into a question by simply putting a question mark at the end. In speech, it is intoned as a question, i.e. you raise the pitch of your voice at the end of the sentence. This is often done with more complex sentences, too.

For example, "I'm leaving XYZ Corporation." "What? You're going to quit your job?"

The idea is that you put a question mark at the end of a declarative sentence to express surprise or disbelief in the statement.

Why it’s wrong

Yes, it’s grammatically wrong. But your way of thinking about English grammar is right in a very profound way: wondering if singular and plural could be exploited to suggest some subtle distinction.

Here’s why it’s wrong. The pronoun their calls for a plural antecedent. Hair is singular. Also, movement is singular, so it has to be grammatically tied to an individual hair. If you tie it to the mass of hairs all together, then it suggests that the mass of hairs all together (what we normally call “hair” with no determiner) has one movement as a whole.

How to do it right

There are ways to use singular vs. plural to indicate what you have in mind. Here’s how you’d do it:

… each of his brown wavy hairs had a movement of its own…

The singular word each gives the singular movement something to attach to, and hairs is plural. This makes it clear that the sentence is talking about many movements, not just one.

You could also indicate separate movement of each strand of hair like this:

…his brown wavy hairs had movements of their own…

You are right that logic often trumps rigid grammar rules, leading a reader to interpret a sentence reasonably when too-strict application of grammatical regularity would lead to clumsiness. But since the language provides a straightforward way to indicate the intended meaning in this case, there’s no pressure to bend grammar.

The inevitable complexity

Their can take a singular antecedent when it stands for a person and you’re trying to avoid indicating “their” gender. However, to many people's ears, this usage sounds sloppy or ungrammatical, or at best informal, because their calls for a plural antecedent. There is currently something of a war going on in the language right now, to allow their to refer to a singular person as antecedent in order to avoid sexist language. Perhaps after that war is won, their will broaden to allow singular antecedents of all kinds, but today such a development is beyond the horizon.