Must Libertarians Believe in Open Borders?

As I write, there are several thousand non-European refugees outside Calais, all trying to enter the United Kingdom. Because they are disrupting travel across the Channel in the main holiday season, the British media has no choice but to report on their presence, and to keep reporting. Their presence is followed by the British public in part because of the disruption, but mainly, I think, because of what they visibly represent. Britain, together with every country like Britain, is faced with an inward movement of peoples no smaller in extent than the mass-emigrations from Europe that settled North America and Australasia, and perhaps as great in its effects as the incursions from across the Rhine and Danube that transformed the Western Provinces of the Roman Empire. We face a mass-immigration from the Third World that may eventually double or treble our populations, and that will, by inevitable force of numbers, make us minorities in what we have so far considered to be our homelands. What have we, as libertarians, to say about this?