But they all knew it and felt it and the robin and his mate knew they knew it.
At first the robin watched Mary and Colin with sharp anxiety.

For some mysterious reason he knew he need not watch Dickon.

The first moment he set his dew-bright black eye on Dickon he knew he was not a stranger but a sort of robin without beak or feathers.

He could speak robin (which is a quite distinct language not to be mistaken for any other).
To speak robin to a robin is like speaking French to a Frenchman.
Dickon always spoke it to the robin himself, so the queer gibberish he used when he spoke to humans did not matter in the least.

The robin thought he spoke this gibberish to them because they were not intelligent enough to understand feathered speech.