Yellow-rumped Flowerpecker Prionochilus xanthopygius is a common Bornean Endemic in North, North-east and Central Borneo from sea level up to 1,100 meters. Its diagnostic yellow rump is not easy to spot in the field as it is always hidden beneath the folded wings, so field identification in Borneo is by spotting the presence of white malar stripe, which is the diagnostic feature of the similar but much rarer Crimson-breasted Flowerpecker Prionochilus percussus, which has no yellow rump but having white malar stripe.
Like typical Flowerpeckers, this is a small bird (9.5 cm to 10 cm) which forages for fruits, nectar and small invertebrates at all levels of the forest. Apparently it is more numerous in logged forest than unlogged forest.
Susan Myers is the only author on birds of Borneo who describes the juvenile of this species in her fieldguide to the birds of Borneo, in which she writes; "Wings and tails blackish-brown, rest of upperparts blueish-grey, underparts greenish-grey, rump dull yellow; bill pinkish."
This is the picture of a fledging bird from Tawau, it was photographed when following the adult birds to feed. Too bad I did not get the picture of them together.
You can see that it is more olive green all over, wing showing blackish primaries, duller underneath, with orange bill and blackish front half of the bill, dark grey feet, dull yellow rump(can be seen in the second picture).
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