Photo 6: 4cy LBBG, July 15 2001, IJmuiden, the Netherlands. 

Less than 1% of the 3cy LBBG show a mirror on 2^nd generation p10. Here a 4cy LBBG is depicted, showing what might be expected in 3cy LBBG with a mirror on p10: the small white mirror in such cases is confined to the centre of p10, not reaching the outer and inner web's feather edge. Apparently, 4cy LBBG not necessarily have a large mirror on p10 either, as can be seen in the picture. To distinguish 3cy from 4cy, clues have to be found in the bare part coloration (much more advanced and adult-like in 4cy) and the pattern on old coverts: the greater coverts are very bleached and worn, yet the pattern is pretty adult-like. Last winter this individual probably moulted most of the coverts to adult-like grey feathers, common in autumn 3cy, so now this bird is probably in 4cy.