Richard Dobbins, advises - "Yes, Chiffchaffs are short distance migrants, juveniles have no post juvenile flight feather moult. Adults moult flight feathers after breeding. By this time of year, Spring, all flight feathers are worn..... they are all last years feathers.
Willow Warblers look smarter because they under go a full moult in Africa...they need good feathers to fly back. Adults also moult all feathers after breeding to have good feathers to fly to Africa with." Thanks Rich, interesting stuff.