Hello Roger (and
all)
I can't get an idea from your photos or comments about
relative size of the two birds. This would probably help in at least identifying
their sex and so hinting at context of their behaviour. Paul McDonald (from
Canberra) worked out the plumage stages at least at Werribee Vic, years
ago. Sadly this was after the HANZAB for the raptors was done, so is not cited
therein. This text below (in red) is what he sent to me 11 years ago. I would
not wish to add to that, apart from to say that the dark bird in your photo is
looking very (I mean unusually) neat. This species so often looks
scruffy.
Fledglings have a
much different plumage to adults, being a very dark chocolate colour on the back
and generally light-medium brown feathers with chocolate blotches on them
throughout the belly and breast. Lighter areas on the head which are normally
light brown in adults, eg the spots on the back of their heads, chin, pale
eyebrow and forehead are a buffy yellow colour in fledglings and juveniles (I
use this term for 0-first moult). Fledglings also have buffy yellow undertail
coverts and have bright blue eyerings, legs and ceres. This is a variable trait
in adults (males range from bright yellow right through to a light blue, females
generally always half pale yellow/light blue at best), but once you are used to
the colour of adults, juveniles are nearly 'glow' blue. They also lack cream
spotting on the sides and inner underwing coverts. There is a good paper by one
of my supervisors David Baker-Gabb in an old Emu that has good pictures, except
the fledgling there has white (adult) undertail coverts not buffy yellow by
mistake (read the text closely and ignore the picture). Simpson and Day also
have a picture of a first year bird that from memory is OK.
Birds that are one to two years old are a little
trickier, especially in worn plumage, but basically are not spotted (see above)
and lack distinctive adult underwing and breast/belly patterns. When moulting
these birds re-acquire the same dark choc feathers of fledglings, although in my
(very limited!) experience this is not quite as dark. Bare parts are still blue,
but less noticeably than in the first 12 months and can probably only be really
noticed in the hand. Undertail coverts appear to be less yellow than birds in
their first year but my gut feeling is that this is not reliable and virtually
impossible to pick in the field. These birds appear blotchy and 'untidy' on the
front, whereas adults generally have clean light and dark areas.
Hopefully Santa will bring me a scanner for Xmas, if you
are interested I could send some Jpegs then. I stress that the above dogs
breakfast is what I use to split them, and is based upon a small population in
one area of AUs, not vast experience with BFs all over the place, so it may only
hold for Vic birds. It definitely holds for the Werribee population
though.
Thanks for taking an interest,
Paul
Philip Veerman
24 Castley Circuit
Kambah ACT 2902
02 - 62314041
