... I noticed this over on the South Wales blog. I also noticed only team members can leave a comment so, given no other bugger had responded and at the insistence of my barely containable, bordering on neurotic pedantry, I felt compelled to answer Mr. Smith's queries...
1. "Do all first winter blackcaps have brown heads?"
No, following completion of the partial post-juv moult (late summer/early autumn), 1st-winters have more-or-less adult coloured crowns; however, a small number of brown feathers are often shown by 1st-winter males but usually go unnoticed. Extensively 'brindled' birds, as shown in the post, are the extreme end of the spectrum.
2. "If so, sexing young Blackcaps pre head moult is almost impossible and would account for the higher proportion of females (?) reported in the 2nd winter period (Oct-Dec)?"
Moult of head feathering is part of the post-juv moult mentioned above. I'm not aware of a "higher proportion of females (?) reported"? [NB. '2nd winter' generally refers to a bird's second winter not the Oct-Dec period, males showing some brown in the crown would almost exclusively be in their 1st-winter.]
3. "Adult males with black heads in this period are OK - but maybe not all 'brown headed' Blackcaps are females?"
Basically by early autumn all Blackcaps have the 'correctly' coloured head according to sex. However, during exceptional views or with photos you might still be able to age them, e.g. if you can detect a moult contrast in the greater coverts (the post-juv moult only replaces a varying number of these, leaving two generations of feathers which differ slightly in structure, colour, wear and length) or ascertain tail feather shape (juv tail feathers, which aren't replaced until completion of the birds first breeding season, are generally more worn and less rounded at the tip than subsequent generations, beware wet adult type feathers).
PS. If any of the above reads like tutelage in egg sucking, then I apologise, but, take a bow, because that makes you a granny.