Booted Warbler Iduna caligata breeds Finland to Mongolia, in China in northern Xinjiang. Winters India. Steppe habitats for breeding, where seeks invertebrates in dense, usually low vegetation, or on the ground. Twitches tail erratically, not dipping it (as Eastern Olivaceous Warbler I. pallida); often flicks wings simultaneously. May also be mistaken (particularly in worn plumage) for paddyfield warbler, Blyth’s reed warbler, or a plain-colored Phylloscopus warbler. Monotypic. The smallest Iduna, with a round crown (that can be raised in a crested appearance when agitated) and short bill. Upperparts slightly warmer than those of Sykes’s, being olive-brown (fresh) to greyish-brown (worn), with some contrast against blackish alula, exposed primaries, and centers to tertials. Has prominent whitish supercilium (more distinct than that of Sykes’s, shorter than that of paddyfield) extending to ear coverts (that are usually darker than in Sykes’s), above which it tapers; supercilium sometimes demarcated by dark lining above (mostly in front of eye). Dark loral line usually stronger than in Sykes’s; white broken eye-ring. Underparts whitish with a buffier tinge, especially on breast sides and flanks (throat and undertail coverts often appears whitest), than in Sykes’s. Square-ended tail with white sides, shorter than tail of Sykes’s. Primary projection short, but longer than in Sykes’s. Yellowish bill, which often appears slightly down-curved, has grey-black ridge above, shorter than that of Sykes’s (emphasized by bare base of bill in Sykes’s, often with a darker smudge at tip (usually absent or faint in Sykes’s). Tarsi brownish-pink, the toes often darker and greyer, giving “booted” appearance. Juveniles, however, have greyish feet (as in Sykes’s). Square-ended tail with white sides, tail movement, finer bill (with broad base), and greyer upperparts distinguishing features against Blyth’s reed and paddyfield warbler. Siberian chiffchaff has weaker and darker bill, blackish legs, and longer supercilium. Song at start slow and quiet, increasing in tempo and volume; a sweet twitter, softer, slower and bubblier than song of Sykes’s; has dry “chreck” call and rolling alarm call. — 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.