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It can be hard to tell sub species from the tail at that age.

A Solomon Island is approx 35cm long, RS 37 to 40cm long.
How much does he weigh? SI can be upto 350 gram, sometimes a little smaller at that age. RS are heavier.

RS also have brown on their beaks. SI either have very little brown or none at all.

Sonja can you post a pic of him?

At about 2 to 4 years is when they have their full feather & not look so scruffy LOL. 2 years for SI, 4 years for larger subs.

No brown at all on the beak may be a SI. SI are also becoming increasingly popular in Australia. It used to be RS. If you look at my avatar, Eddie is SI with no brown on beak. Their bottom of beaks do go lack & as they get older their tops go candy corn. Eddie's was a dull yellow at 6 month with no brown at all. The brown is on the upper beak (if there is any) It can be completely brown or just patchy.
This is a RS with brown on his beak http://static.flickr.com/44/162660747_566418c3be_t.jpg
but SI can also have the brown. As far as I know the SI are the only ones who sometimes don't have brown beaks.