Originally Posted by Jon Turner
I did partially think I should have made the original question one of both age and sex. I didn't have the courage to mention gynandromorphism, but is it possible here?
I'm not sure we could propose gyandrotrophism for this bird as there does not seem to be the usual left-right division that typifies that condition.
I see no reason to reject the idea of older female Blackcaps getting some degree of black in the crown feathers. An increasing ratio of male hormone is presumably the reason why the brown feathers of juvenile males are replaced with black (and perhaps a little brown) during the pj moult at 1- to 3-months-old. If that same change of ratio occurs in females, but much more slowly than in males, or if production of female hormone declines with age as it seems to in mammals, then who knows...
One of the great things about science is recognising there exists stuff we don't yet know. It not only keeps us on our toes, but it also stops the subject from becoming boring
What a pity there is no known way of ageing most passerines after the first post-breeding moult takes place in their 2nd calendar year of life.