Dear Koel,
All your pics are of Arctic Skuas. On shape and size they all have relatively long slender bills [Pomarine is heavier], long slender wings with the hand as wide as the arm [Pomarine has broader arms], and lack the thick-necked/barrel-chested/pot-bellied appearance of Pomarine. Where pointed tail feathers are visible on Bird 3, these are diagnostic of Arctic.
For ageing, note all pics taken mid-August in Japan. At this time, adults would still be in summer plumage as their moult does not begin until later in autumn, and you would expect to see juveniles moving south with older birds.
Birds 1 and 2 are both dark phase juveniles. They have pale bills with dark tips which shows they are not adult, and either no tail streamers [Bird 1] or only the beginnings [Bird 2]. They do not have distinct unfirm dark caps and have paler neck bands [esp Bird 1] which are typical of juveniles.
Birds 3 and 4 are older. Bird 3 is pale phase whereas Bird 4 is intermediate. Both have plumage patterns close to adults with distinct dark caps and pale collars. However, their underwing coverts are not uniformly dark and they appear to have pale bases to the bill, both indicators of younger birds. Also from above they have paler upper tail coverts which again is a sign they are not yet adult summer [note adult winter birds have paler uppertails, but as noted above it is too early for adults to be in winter plumage]. Bird 3 has distinct pointed tail streamers which are well developed but not yet full length. Bird 4 shows no tail streamers - perhaps they have been broken. I consider both to be second summer. They are not first summer which is much more like juvenile and would show contrasting ages of feathers on the wing at this time of year.
Hope you enjoyed the trip. The Hokkaido ferries are a great way to see a wide variety of north Pacific seabirds.
Mike Chalmers
