A present from my Swallows.

We came to this house in 1976 and to my delight there have always been Swallows nesting in our old stable building. This year the pair have already raised five young which have fledged safely. The female has been on a new nest since her first brood fledged and today I found this tiny egg-shell below it showing there is a new occupant in the nest. The egg-shell is about the same size as a 5p coin yet the little bird which hatched from it will set off for Africa in a few weeks time on its migration. How is it possible that such a miracle can happen from such a fragile little egg-shell?