Last week, in the Botanical Gardens, I bumped into an unknown plain-coloured phyllosc (no wingbars) with olive-brown upperparts, olive fringes to wings and tail, buff wash on flanks and breast. The bird was feeding at eyes level among small trees and also on top of a wall. It did not call.
It was clearly not a Dusky Warbler (always mid to dark brown above; no olive is ever apparent in the upperparts). At first, I thought it was a Radde’s - a bird I am now quite familiar with - but the jizz and some color pattern details did not match. Its clearly demarcated pale throat contrasting with the brownish breast first drew my attention; this was very obvious in the field (in frontal view). The throat also appeared somewhat puffy. Moreover the bird did not show the large-headed, somewhat bull-necked appearance of the Radde’s.
Once I got home and examined my pictures, I also noticed the striking broad and rather uniform supercilium (lacking strong contrast between fore and rear parts, only very faintly browner to the fore), the pale orange legs unremarkable in size (thick tarsi in Radde’s), the lack of contrasting peachy vent. The bird also show yellow streaks on the underparts – but Radde’s can also be streaked; so that is not a clinching ID feature.
After checking the photos against Brazil (2009) and especially various internet based resources, I identified it as Yellow-streaked Warbler, an uncommon winter visitor in Vietnam, apparently only recorded from the north of the country (and already recorded at Hanoi by Falk and Simon few years ago).
It seems that Yellow-streaked is a particularly difficult bird to tie down unless you hear the call (a metallic bunting-like tzic). That is why I'd be happy to hear experienced birders opinion, especially those who have already observed this species.
Here a bunch of shots (sorry for the "concentration camp" atmosphere):
Note the breast band (most easily judged when the bird is in shade, as sunlight tends to work out the colour ) contrasting with the pale throat, the yellow streaks on flanks, the broad and rather uniform supercilium.