In W.H Auden's poem, "Musée des Beaux Arts,” what is being alluded to by the phrase "the miraculous birth" (line 6)?
The poem “Musée des Beaux Arts” by W. H. Auden describes a painting by Pieter Bruegel (the Elder) called "Landscape with the Fall of Icarus" that Auden saw in the Musées Royaux des Beaux-Arts de Belgique in Brussels, Belgium. It hangs in a gallery with other paintings by Breughel portraying Renaissance Flemish peasant life and traditional Biblical themes.
The particular phrase occurs in a section of a poem contrasting the sacred, extraordinary, or miraculous with the mundane: the "aged are reverently, passionately waiting / For the miraculous birth" with "Children who did not specially want it to happen." Auden explains that "the old Masters ... never forgot/ That even the dreadful martyrdom must run its course..."
In this context, the miraculous birth is probably that of Jesus, and the "aged" the three Magi, as this was a common theme in the paintings a visitor to the museum would have seen. The subsequent reference to a "dreadful martyrdom" supports this reading.
The ancient Greek story of Daedalus and Icarus is not religious; Daedalus is portrayed as a clever inventor, and Icarus' fall is due to what we would now call a pilot error (flying too close to the sun on wings held together with wax). Nonetheless, it still is an extraordinary feat, especially if you imagine it occurring in a period where modern flight technology had yet to be invented.
The point Auden makes is that no matter how extraordinary these events, the fall of Icarus from the sky and the miraculous birth, most people continue their ordinary lives without paying much attention to them, for even if a miraculous birth occurs, or a boy falls out of the sky, farmers still need to tend to their crops and ships must sail to their destinations on schedule.