More important than what it is,
for information purposes, is my suggested approach (put forward for
discussion).
First, adopt a common
terminology. Are we going to use ‘juvenile’ for any pre-mature bird, or,
as I suggest, reserve the term for true juvenile (first plumage) birds and use
eg ‘ímmature’ for birds from second plumage and before
maturity.
Secondly, get the most
informative image. Is the below the best available?
Thirdly, look for signs that the
bird is a true juvenile (first plumage) bird. The below is evidently not
in that category.
Fourthly, examine the bill for
signs of yellowing. (Not entirely clear in the
below.)
If the bill is entirely black,
the bird could be a 2/3 year bird of either sex or a female between 2 years up
to and including maturity.
If the bill has signs of
yellowing it is a male from 4 years to short of maturity.
Information about relevant
behaviour or help from someone with expertise on plumage differences might be
able to advance the matter further.
By the way, I sent my earlier
message to Danny Rogers who added the comment:
“ I would incidentally
be surprised if males were completely consistent in the age at which plumage and
bare part changes occur - in other species of bird in which long delayed
maturity occurs there usually turns out to be some variation - in part probably
driven by social and territory status, but perhaps with lots of other
interacting causes too.”
