Swallows passing through

In the last four weeks we have been catching swallows in a local reedbed on one evening per week and have now processed 330 birds.  The first three catches, each around 100 birds, were comprised almost totally of juveniles with the initial catch mainly recently fledged birds.  Each week has seen the plumage of the juveniles advance and last week more birds near to completing their post juvenile moult were caught. 
In the first three weeks we caught very few adults (one or two each week) but yesterday with only 22 swallows caught there were 4 adults, all females, perhaps indicating that the breeding season for the adults is just about completed. 
Perhaps more surprisingly, even though the reedbed has been an attractive roost for swallows every evening, none of the birds has been retrapped - they have all moved on quite quickly. 
The islands have recorded a light swallow passage to date and my notes suggest that the first third of September is peak swallow migration time but these ringing notes suggest that the passage has been going on for the last month although perhaps in smaller stages and in smaller groups which we hardly recognise as migratory movement.