Simon, 'size' difference is very small and as you know difficult to
estimate. How did you estimate size and was that the only character that you
used to ID the bird? No wonder you thought the submission would be 'easy'.
Below is a detailed set of reasons for the ID.
Mike
Earlier photos on Birding-Aus were Grey-streaked, but this one is definitely
Dark-sided in my opinion since the diagnostic dark tips to some of the
undertail coverts show in the side-on photo. They are inconspicuous but this
is not at all unusual. Grey-streaked never shows these marks.
Other characteristic features are:
- the short bill length (very similar to the Cocos bird). In Grey-streaked
the bill is longer and stouter (as seen in image of adult birds posted
earlier)
- slightly asymmetrical eye-ring (though less pronounced than in Cocos bird)
is typical of D-S.
- marks on breast and sides are diffuse and background colour is dirty, not
clearly defined streaks on a white background as in Grey-streaked. The way
in which the dark marks on the breast join up under the throat to form a
"necklace" is typical of Dark-sided.
- primaries appear to reach no further than undertail coverts as seen in
second photo (in Grey-streaked they extend further). I measure the tail
projection beyond tips of primaries in second photo as about 15.1% of the
overall length of the bird (as measured in a straight line from crown to
tail tip). This is very close to the average (15.4%) I measured for
Dark-sided from many photos on Oriental Bird Club website as part of
submission on the Cocos bird. For Grey-streaked it averaged 12.9%.
- dark lores with pale loral stripe but no pure white is typical of D-S.
Rear portion of lores shows the dark triangle mentioned in Alstrom and
Hirschfeld (1991) as being characteristic of D-S (and seen in the Cocos
bird). Dark marks in pale portion of lores is also said to be characteristic
of Dark-sided. Pale loral line does not extend to top of eye (also
characteristic of D-S).
- buff showing in tips of some wing coverts (characteristic of D-S).
The lack of a pronounced half-collar is unusual although I have seen images
of D-S that lack it. I have an updated version of summary id table (with
references) from BARC submission if anyone wants it.
Regards
Neville
|
