Not clued up, but as I understand it two key issues in relation to migrating wild birds are:
(1) can healthy birds carry the virus (because if they catch it and die, they won't be able to spread it by migrating)
(2) in cases where wild birds have been found dead with virus (such as Bar-headed Geese and others) did they catch it from poultry (rather than vice versa).
As for the virus itself, it is only a threat of the sort you desribe if (in addition to it being transferred to and carried by healthy wild birds, which I believe remains unproven) it mutates into something which can readly be transmitted from human to human. So far, if human to human transmission has happened at all, it has been in pretty exceptional circumstances.
By the way, if anyone has a link to the text of Ben Bradshaw's (Env' Minister) recent statement on this issue, I would be interested to see it. According to press reports one comfort factor cited in it is that the birds potentially carrying the virus migrate east to west, whereas "ours" migrate north to south..