third calendar year: January - April

During April only the very first of 3cy birds return to the colony. The first arrive around mid-April, the majority around mid-May in the Netherlands. They join the adults arriving as early as possible to find the best breeding spots in last years colonies. The counted maximum number of 3cy’s estimates 100 (mid-May), on the total of 1.500 LBBG present at the Maasvlakte, the Netherlands. Higher numbers can be found on the wintering grounds (Portuguese coast and in S France) or at the stop-over places (e.g. Pas-de Calais). 

moult stage in typical 3cy LBBG: 
It is important to distinguish between 'typical' 3cy LBBG and advanced birds. The activity in moult during April is virtually zero in typical LBBG, what has long been considered typical graellsii. In these typical birds, there has been no moult on the wintering grounds and returning 3cy birds in spring show more or less the same plumage as the plumage of 2cy October birds. They have a worn second generation tail, i.e. a tail with a black, broad sub-terminal tail-band, which has been required last autumn in the complete moult. Some birds will start or have started moulting some of the tail-feathers by April. The primaries are slightly worn at the tips, but still all second generation. 
Otherwise, these typical 3cy birds still show brownish second generation wing-coverts (greaters and lessers at least) while upper-parts and bare parts look very adult-like, resulting in the well-known "grey saddle" in 3cy LBBG in spring.

moult in advanced 3cy LBBG: 
By April, groups of 3cy LBBG increasingly include advanced birds, most obvious advanced in the upper-parts and wing-coverts. It's not uncommon to find 3cy LBBG with recently replaced wing-coverts in April, indicating that these feathers have been replaced on the wintering grounds, during the last months. Such birds may show complete adult-like upper-parts and the dark grey-tone of the new scapulars and coverts suggest they may origin from intermedius colonies. However, they may also return in Dutch colonies, as this image shows. It is worth checking the tail, secondaries and primaries in such birds, as advanced partial moult in the upper-parts may correlate with moult in these tracts as well. 

tail-feather & primary moult in advanced LBBG:
Most commonly seen at this age now, is a worn second generation tail. But remember that quite some 2cy LBBG return in NW Europe with second generation tail-feathers in spring, and renew these tail-feathers in a second wave in the complete summer moult. Consequently, such birds may already show an almost complete white tail by October in second calendar year (see here). Therefore, it is possible that 2cy LBBG leave NW Europe with already a white tail and show white tails in spring again.

3cy LBBG in spring may include tail-feathers in the partial spring moult again, and may still be actively replacing rectrices in April (see e.g. this image). About 30% of the 3cy LBBG have moulted one or more tail feathers by the end og April, giving them the blocked pattern of white (third or fourth generation feathers) and partial black tail-feathers. The partial moult involves rectrices randomly, creating an unpredictable black and white tail in spring.  

At the end of April only the very advanced birds have started the complete moult and have shed P1. The ordinary moult-sequence of the secondaries start when P6 is almost fully grown, by mid-summer. But here again, primaries and secondaries may be included in the partial moult on the wintering grounds. This image nicely illustrates arrested primary moult in spring 3cy LBBG. On the wintering grounds, partial moult must have been very extensive in the upper-parts, as almost all wing-coverts are very adult-like. In this extensive moult, the inner primaries were moulted as well, just as most of the tail-feathers.

In this respect, the western taxa intermedius and graellsii LBBG may resemble eastern fuscus., which very commonly replaced remiges and rectrices on the wintering grounds.