Comment by anonymouskimmer

I believe the rationale is that during the process of infecting a flock of birds the virus would be exposed to pressure that would encourage its mutation, especially as these birds begin to successfully fight it off. The current avian H5N1 only needs a couple of mutations to spread human-to-human pretty well.

So the current culling of entire flocks is seen as a means of nipping any of these mutations in the bud.