Interesting Siskin

Today I have just passed the 200 mark for new birds in 2014!  A good session on Leith Hill with SW, PS, DC and DS produced 35 new birds including a Brambling, a Marsh Tit, 9 Lesser Redpoll (including a British control) and a Common Redpoll (below).

Although the most interesting bird was a 1st year male Siskin with an unusual moult.  As a rule with Siskins adults mould all feather after breeding whilst juveniles moult all body feathers, most if not all wing coverts (except primary coverts) and regularly the alulas and tertials.  But this bird has decided to be different and having done all of the above has then also moulted all of the outer primaries from 4 to 10 on both wings (below)?  Usually when passerine birds moult their primaries the primary coverts moult in sequence with them, but here all of the primary coverts have been replaced not just the ones relating to the replaced flight feathers!  Why has this one decided to do something different?

You can see the old juvenile feathers are brown, worn and have pale yellow markings, whereas the new feathers are black, fresh and have bright yellow markings.