I'd read recently that it was more to do with costs than bird flu. As Rowan said, feed prices in particular have skyrocketed, but the supermarkets have been unwilling to pay the extra. Hence supplies have been held back. I noticed some shelf prices going up before Christmas, so maybe the supermarkets are paying a fairer price now.
Its because supermarkets keep the prices paid to farmers so low that they are cutting production - nothing to do with bird flu (but of course it doesn't help as it adds to their costs).