Yes birds can gain immunity to avian flu and I recall that in the low pathogenic H7N3 case in Norfolk last year, many of the free range birds had recovered.
But the problem with the H5N1 high pathogenic strain is that mortality is very high with 70-90% of birds dying within hours of catching it. Allowing it to take root in the UK could lead to a similar problem as seen with cattle TB with a wildlife reservoir reinfecting flocks, particulary in ducks which dont exhibit the symptoms and remain carriers.
Wild ducks are suspected of having a role in the asian outbreaks.