Mike,
The birds oversummering in Mai Po or other wintering sites should have 'dirty' looking because they have more worn feathers. The birds born in this summer just have their feathers for 3 months and it could still look very fresh.
An extra point should also be made that mould of BFS, especially those of young birds, is still poorly studied. Photos taken in close range to show details of wing and tail of the BFS could provide some information of the moult of the BFS. So, I would like to encourage members to post pictures of BFS in such condition and this can help us to understand more to this endangered species.
Thank you very much
Tung