Sand Martin movements

We received a batch of bird movements today which included 6 birds that were caught in autumn in France on migration. 5 were ringed as juveniles on the river Lune during June and July 2011. The most interesting one is L334113 which was ringed as a breeding bird in July 2010, recaught in June 2011 at Whittington and then caught in Charente-Maritime, France in mid August on migration. I'm getting increasingly interested in how long after fledging the juveniles leave the colony and then how long before they start a southwards migration. From ringed birds we catch it is clear there is a period after fledging but before migrating south juvenile Sand Martins roam, possibly finding somewhere to breed the following year, and then move quite rapidly south. I hope with the 15-20 birds that are caught on their southward migration from the Lune each year we'll soon be able to look into this.