But why the robin? The origin of the robin as a Christmas bird dates back not more than 100 years, and is rather mundane. It is all the fault of the Post Office.
In the middle of the last century the Post Office dressed its postmen in bright red coats of a colour to match the official red of the pillar boxes. Because of this striking uniform, the postmen themselves came to be known colloquially as "redbreasts", and there are references in the novels of Anthony Trollope* (who was himself a postal official) to "robin postmen".
* (Anthony Trollope (1815-1882) - British novelist; described in his novels the life of provincial England.)
So it was inevitable, when the Christmas card first came into general favour around 1860, that robins should figure prominently in its decoration. Almost all the early cards showed a cheerful robin redbreast, often bringing the welcome Christmas mail in its beak, or sometimes actually knocking on the door, just like the postman himself.
The scarlet-clad postmen are now forgotten, and their present-day successors wear a sober red-piped blue, but they still carry vast loads of robin-adorned cards each festive season.