LPAI viruses circulate among wild birds, especially Mallards, all the time. As Jourdain and colleagues pointed out in 2010, mallards are not obviously affected by experimental infection - for example they don't lose weight or cease moving around and they don't show any other clear signs of disease (very slight temperature rise for 2-days) after infection. However, the authors note other studies identifying egg production problems and also the importance of further studies on wild populations as opposed to a small number of birds.
Perkins and colleagues showed that ducks, sparrows and gulls tend to control virus replication differently from chickens suggesting that the ducks may have a better ability to mount an immune response. A weaker (less antibody and shedding)reinfection with the same H7N7 virus was still possible even in the presence of an antibody response to the first H7N7 infection. Reinfection by a different virus subtype (H5N2) seems to have been blocked by the antibodies made to initial infection by H7N7; so-called heterotypic immunity (cross-protective immunity to a different viral subtype).
Volmer and colleagues reported in 2011 that ducks infected with an LPAI mount an interferon (IFN) response in their guts. The cells (enterocytes) of the duck gut is where most flu virus replication occurs and its from here that most of the virus shedding originates. This study found the gut was inflamed and that there was some cell death. So, not at all like the respiratory disease the human host experiences. In the cells lining the intestine, the authors found lots of myxovirus-resistant (Mx) gene activity; diverse genes in birds, which may be especially well adapted in mallards to control influenza, that are key to the earliest immune response to virus infections (also important in humans) and particularly effective against influenza virus. Mx proteins trap and redirect flu virus nucleoproteins.
In particular, a Type I (antiviral) IFN response was detected. IFN-g (gamma) gene activity was up, activating important immune cells (T cells or natural killer cells), important for flu vaccines in poultry and humans.
Swayne provides an excellent review of the vaccines used in birds to moderate H5N1 disease ('fowl plague'). The formulation, use and effect of these vaccines are as varied as the influenza viruses themselves. China leads the field in the use of reverse genetics to create the contemporary flu viruses carrying the most relevant immune stimulating (antigenic) bits.