Need an example of how birds can spread a virus? Let's look at West Nile Virus. It arrived in the US, in New York in 1999. The virus update for Sep 26 is 2720 cases with 87 deaths. Idaho has 642 cases, Colorado has 199, California has 215 and Texas has 211 but has the most deaths at 22 reported. This virus has now been reported in the Caribbean, Central and South America. Argentina is reporting cases.
Mosquitoes are the vector. They infect the birds (the reservoir), who then carry the virus further away. As they are bitten by mosquitoes in a new location, those mosquitoes further spread it to other birds and to humans.
We are talking West Nile Virus here, not bird flu. But, for avian flu, we have incomplete knowledge of hidden reservoirs and vectors.