The swifts in this tower have been studied for over 50 years. Every single swift that is born here gets an individual identity ring, though, with such tiny legs it's hard for Professor Perrins to get the ring on. Swifts spend so much time flying, their legs have sort of shrivelled up. Believe it or not, in about a week's time this little bundle of feathers will fall from this nest, spread its wings for the first time and not touch the ground again for two years. They do a few press-ups to limber up but then - apparently with no other practise - the young swift is able to fly as well as its parents and it will need to, because within days it will leave the nest and set off for Southern Africa, 6,000 miles away. From now on, it will do everything in the air: catch food, drink, even mate on the wing. Truly amazing - but what happens at night? Everybody needs to sleep - even little swifts. Well, at this time of night the swifts that aren't nesting start to circle up higher and higher into the evening sky. When they get to about 6,000 feet they nod off! Obviously not a full two-hour doze, or else they'd crash and die. More a series of catnaps as they glide round and around. One swift they found here was 18 years old and they calculated that in its lifetime it must have flown around 4 million miles - that's the distance to the moon and back, eight times!