Red-flanked Bluetail (Tarsiger cyanurus) breeds across the northern Eurasian taiga from Sweden, Finland and Norway to eastern Siberia. All populations migrate to Southeast Asia.
Just like Arctic Warbler and exactly opposite to Willow Warbler and Northern Wheatear, western populations of Arctic Warbler do not migrate to Southeast Asia along the shortest diagonal route. They instead migrate eastwards to eastern Russia and China before moving southwards! An incredibly long journey and one that is repeated when they go back! This also explains why we do not see birds in passage migration through the Indian peninsula.
Red-flanked Bluetail may be another example of a bird species retaining “migration memory”. Despite a significant westward expansion of its breeding range in recent decades, it continues to migrate to Southeast Asia much like Northern Wheatear which continues to migrate to Africa despite a significant eastward expansion.