Manchurian Bush Warbler Horornis borealis shows extreme sexual dimorphism (for a warbler); the males are much larger than the females, the females being close in size to the Japanese bush warbler. Both subspecies of Manchurian breed in China: nominate in Liaoning, Jilin, and Heilongjiang (as well as the adjacent Korean Peninsula and Russian Far East) and canturians in a region ending just south of the Yangtze River and extending west through the Qin Mountains into Sichuan and north to Hubei. Ssp. canturians has a year-round presence in the Yangtze valley; other members winter down the entire southern Chinese coast and as far inland as Yunnan. Some borealis winter on Taiwan. A lowland species, rarely venturing above 700 m, frequenting dense forest and thick scrub. In breeding season, males often sing from an exposed perch, but otherwise keeps well-hidden in the undergrowth. Forehead and crown rufous, usually contrasting noticeably with plain warm brown plumage on the rest of the upperparts and tail; some birds have plainer brown forehead and crown, hence lacking contrast. Long, buffy-white supercilium; blackish-brown eye-stripe. Underparts buffish-white with varying buff-brown wash on breast, belly, flanks, and undertail coverts (strongest on the latter). Bill blackish above, with yellow base to lower mandible; feet brownish-pink. Highly vocal; song, lasting about two seconds, begins with a bubbling whistle and ends with a louder flourish. Call is a repeated “tret” or more sparrow-like “tschuk”. Male Manchurian bush warblers are noticeably larger than Japanese bush warblers; the two subspecies of Japanese bush warbler most likely to be found in China (cantans and riukiuensis ) usually lack distinctly rufous pigmentation on the crown, have colder olive-grey tone to brown upperparts, less distinct supercilium, and more often seem to show less buff underparts than Manchurian. Brown-flanked bush warbler is noticeably smaller than even female Manchurian, also lack rufous pigmentation on the crown, and is drabber. Grey-sided and chestnut-crowned bush warbler are smaller, grey-sided have greyer underparts, chestnut-crowned has strongly contrasting chestnut crown spilling over to supercilium (in front of the eye). — Craig Brelsford
Daniel Bengtsson served as chief ornithological consultant for Craig Brelsford’s Photographic Field Guide to the Birds of China, from which this species description is drawn.