Individual with dark bill, relatively short tail and with dark outer vane on outer rectrices (T6).
This individual shows all the diagnostic characters of a South American Tern first winter. In the photos it is well represented what I saw in the field. Though there is no image of the back of the bird it was alreday grey, this means it has undergone the first molt. This is the difference with a juvenile. That is, this bird is between 6 months and a year old, most probably between 8-9 months old. As I have observed during the molt there is a slight wear on the outer remige (P10), as well as in the tipos of T6. Another distinctive characteristic of non adults of this species is the pattern of wide marks (generally 3) on the tertiaries. This pattern is similar to and may only be confused with that of the Arctic Tern S. vittata, which has a much shorter bill and an even shorter tail. Also the pattern of the back is much more contrasting since inner secondaries and primaries are whiter! Explanation by Lui
Individual with dark bill, relatively long tail and with dark outer vane on outer rectrices (T6).
This individual shows a more advanced stage than the previous one. This is manifested in a more pronounced wear of P10, being in general all primary remiges whiter than those of the previous individual.
Molt is seen to have begun at the tail. T5 (previous to the outer rectrice) is already grown. Though the molting stage of this bird is more advanced than the previous one the nomenclature is the same for both (first winter) since it has not undergone more than half of the molt of the remiges (flight feathers).
Explanation by Lui.