Nest box under eaves

Hi all, This might sound like a daft question, but I just want to put my mind at ease! I'm thinking of installing some sparrow boxes under the eaves of my 2 storey house later on this year (ready for next season). My only concern is, if i'm lucky enough to have successful nesters, what happens when the birds come to fledge? There will be nothing but an almighty drop below them, no branches to land on! I know that traditionally house sparrows will nest in roof spaces and under tiles, but I just have visions of little bodies under the box at fledging time! Has anybody had any experience with a similar set up? If so, how did the youngsters fare when if came time to fly the nest? :)