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The adult Sulphur-bellied Warbler in breeding plumage is dark brownish-grey above, sometimes with not-clear olive tinge; slightly lighter on the rump. The flight and tail feathers are plain brownish, sometimes with the very narrow light edges of outer webs. The underparts are dirty-yellow or olive-yellow (the “beeswax” color); with slightly lighter throat and middle of the abdomen; and buffy-brownish tinge on the flanks and breast; the undertail is more buffy than other parts. The supercilium is lemon-yellow. The under wings and axillaries are buffy-rusty. The coloring of old plumage is dimmer; in fresh autumn plumage the mantle is greyer, the underparts have less of yellow. Juveniles in first plumage have slight rusty-brownish tinge on the mantle; underparts are dirty-yellow. Juveniles are lighter than adults; the upperparts are greyer, underparts have less of yellow and with dark buffy-brownish tinge. Weight 6,5-11,5 grams; male wing 58-68, female wing 55-61 mm.

Biology

The Sulphur-bellied Warbler is common, in places rare breeding migrant. It inhabits the dry stony slopes with the rocks, bushes, scarce grass and single trees at 1900-3600 m in Tien Shan and 500-2000 m on Altai. On migration it visits the deciduous forests, groves, gardens and forest-belts. It arrives in late April – early May, last migrants recorded in mid-May. Breeds in separate pairs at 100-150 m from each other. Nest is built in bush (juniper, honeysuckle, dog-rose, rowan-tree), in grass, at once on low spruce twig; from the dry grass and bush bast strips; and is lined with hair and feathers. Only female builds it for 5-16 days, male helps it very rare. Clutches of 4-6 eggs is in mid-May – end June. Only female incubates for 14-17 days. Both parents feed juveniles, which fledge at 15-18 days old, in end June – early August. One brood per season, repeated breeding after losing of the first nest is common. Autumn migration is early, majority of birds migrate in August - early September, latest birds recorded in mid-September (one bird recorded near Sairam village on 6 October).

References

"Птицы Казахстана" том 4. "Наука", Алма-Ата. 1972. Gavrilov E. I., Gavrilov A. E. "The Birds of Kazakhstan". Almaty, 2005. Э.И.Гаврилов. "Фауна и распространение птиц Казахстана". Алматы, 1999. А.Ф.Ковшарь "Певчие птицы". Алма-Ата, "Кайнар", 1983.