There are no ready sources that I know of (asked this question by science journalist yesterday; he’s looked at some migration routes etc, but likewise found no ideal source of info). I earlier gave partial answer re flyways in email to same journo, which I posted to thread at: Migration Flyways V briefly here (I’ll add a bit more to that thread):

Nigeria is on migration flight paths, but for birds from Europe – ie west of areas known to have H5N1 before birds migrate. Few areas of land on the planet are not on flight paths. You live in the US, so you are on or near flight paths (with higher densities over Missouri than over many places). Wild birds likely migrate right over your house. But you’re not on flight paths of birds from areas known to have H5N1