Alpine Swifts

I've just been admiring the aerial ballet of a flock of Alpine Swifts above the oak trees on the ridge up behind my house. They're darting and diving incessantly, to capture edible insects. These elegant birds, which reappear once or twice a year for short spells at Gamone, have always fascinated me, because ornithologists claim that they simply never alight anywhere on the surface of our planet during their entire existence. I've always found that story hard to believe... like the tale of the 5th-century Syrian ascetic Simeon Stylites who is said to have resided permanently on top of a stone column. What I mean to say is: How could ornithologists possibly keep track of individual swifts, day and night, to make sure that they never land anywhere? That would be even more difficult, to my mind, than parents trying to keep track of the nightly movements of their teenage offspring.