Let's summarise the evidence. The H5N1 virus emerged and acquired its nastiness in poultry; there's no dispute about that. The trade in poultry is also overwhelmingly responsible for H5N1's spread in east Asia. It was a different story, however, when the virus, having killed wild geese at Qinghai Lake in China, spread across Russia and into Turkey, Ukraine and Romania. The strain found in all these places is identical to the one first seen at Qinghai, never in east Asian poultry, which strongly suggests that it was not carried west by poultry. Maps of the outbreaks look a lot like the migration maps of dabbling ducks - long-distance migrators that tolerate the virus.
Since then the evidence has become even clearer. There have been outbreaks in western Europe almost exclusively in wild birds, and in all cases of bird flu west of Qinghai the virus is identical, suggesting it has found a carrier where it is more stable than in chickens.