I had some extremely interesting and productive correspondence about an interesting Radde’s Warbler in the UK last autumn with Martin Garner. Ian Lewington checked some specimens at the British Museum at the time and Martin forwarded me his comments.
Based on this and my paper in the 1994 HKBR I’d suggest the following are the main criteria for identifying a Yellow-streaked Warbler:
• A metallic bunting-like tzic call (a soft quip in Radde’s, similar to one of the calls of Russet Bush Warbler)
• A dark brown ground colour to the breast/belly forming a darker breast band (typically whitish on Radde’s, and rarely showing a breast band)
• Pale throat more clearly demarcated (less so in Radde’s)
• Supercilium often whiteish behind the eye (variable in Radde’s)
• Supercilium often more distinct in front of eye
• Upperparts more brownish on average (more olive on average in Radde’s)
Bill structure is a difficult feature to use as many Radde’s (presumably females) have smaller bills.
Note also how the bill structure varies in the photos of this bird, looking very thin in one photo.
On balance, I would say that this is a Radde’s Warbler.
