Then after a while longer, when it has become ragged and worn, it gets loose in the skin and drops out--as I am sorry to say some of my hair is doing already.

That is what we call _moulting_." "I know about that," interrupted Nat.

"It's when hens shed their feathers.

But I didn't know that it was moulting when people grow bald." "It is very much the same thing," said the Doctor, "only we don't call it moulting when people lose their hair.

But there is this difference.
Birds wear out their feathers much faster than we do our hair, and need a new suit at least once a year, sometimes oftener.