Juvenile dispersal afoot already?

I was a little surprised to see a young Common Whitethroat in the garden for a short period. There's certainly been no immediately local pairs around so one imagines its wandered off on becoming independent. It won't be long before the first birds begin to move away altogether. Cuckoos suddenly seem to be both quiet and less obvious, suggesting , perhaps, some of the adults are already moving out. Contrasted against this I had a House Martin gathering up mud from a "streamside" intent on nesting on a nearby farmhouse where no birds had been present previously. Are these birds returning from the farthest point of their wintering quarters, and arriving later onto the breeding grounds due to the distances involved, or failed breeders or victims of nest collapse even?
An interesting conversation with a colleague on the Oa, where the Tree Sparrows have again turned up in force and are breeding at a couple of locations. These birds disappeared during the winter, but have dutifully turned up again in May. Both of us had had similar experiences with House Sparrows, with birds suddenly appearing late, including a youngster with a female here, but with no regular evidence of presence beforehand. Whilst they're pretty resident features in the villages, their distribution can be patchy outside of these "centres", with small populations at isolated farms and no birds necessarily at locations in between. More work needs to be done on what they do in winter, whether their range temporarily retracts to the villages or whether they remain in situ.