just emailed this to a science journalist; may be of some interest:
I think exact data could be tough to come by: partly due to relative dearth of info; also as flyways are rarely tightly defined. Some species have tendencies to follow flyways more strongly than others - such as birds of prey, which typically heavily dependent on thermals, so can avoid sea crossings if possible. Some waterfowl, too: I've seen geese travelling along east coast of China, for instance. For some/many species, can be more "broad front" migration.
No nice, tight lines, then. But that doesn't mean all is random. Instead, can have lines that broadly show what's known - from observations, ringing studies, and a smattering of radio tracking studies. If do these for groups of birds - waterbirds, say - get the rather fuzzy "flyways" you'll have seen already.
But move towards species' levels - and best if select some key species - and should find clearer pictures. Especially if start adding some timings, for when they're on the move.
Bar-headed Goose, say: inc Qinghai (and Chany Lake) - India movements. - for this and other Qinghai birds, should become clear that they are indeed sedentary, maybe even with some moving south, by July, when Russia was hit by outbreaks.
Common Pochard has become another species in news - mainly, it seems, as it's about the only one people have noted that breeds in bird flu hit areas, migrates to west Europe. For this, I think picture will be more complex, as a common species, and widespread. Some reach Hong Kong each winter; so I guess the "flyways" would really be just lots of lines radiating out towards winter areas, from east Asia right across to western Europe. (Pretty typical of waterfowl overall.) Again, I'd figure movements should be around now; by November, becoming fairly settled in winter areas. (Many bird movements pretty much over by around end of November; protracted cold weather during winter can cause further movements - such as some ducks etc leaving the continent for UK, but these aren't annual.) [even now, many shorebirds have ceased migrating - some that passed through HK earlier likely in Australia by now; it's the more cold tolerant birds that are on the move at present, with real hardy ones such as geese and swans showing tendency to move as lakes freeze]
Timings remain important; I showed for 2003/04 they were massively at variance with bird flu spread in e Asia. Romania/Turkey fit bird migration timings, but doesn't prove wild birds responsible - why are only these two places in Eurasia apparently affected (that we know of; Nial's written of maybe a million ducks n geese now in Korea, with no problems seen as yet)? What of trade - where do farmers obtain their birds?; I haven't seen answers, and believe lack of focus on such questions, and wild birds as maybe red herrings, could exacerbate problems.