The swallow is a type of bird, varieties of which are found throughout the world. The European swallow migrates seasonally, while the African variety does not.
In Monty Python and the Holy Grail, King Arthur suggests the empty coconut shells his servant Patsy uses to make horse hoof noises could have been brought to Britain by migratory swallows, a theory which is debated at length by two guards at a castle where Arthur had hoped to recruit members of his court at Camelot.
Later, at the Bridge of Death, the Bridgekeeper asks as his third question to Arthur, "what is the airspeed velocity of an unladen swallow?" When Arthur asks the old man whether he means an African or European swallow, the bridgkeeper is unable to answer, and is thus launched into the chasm. When Sir Bevedere asks how Arthur knows so much about swallows, he replies, "you have to know these things when you're king."