I wonder how related this is to H7N9 being a low pathogenic avian influenza (LPAI) in birds compared to H5N1's highly pathogenic course in birds. H7N9 can, in theory, stealthily sweep through flocks without setting off veterinary alarm bells (human cases acting as the "sleeping" canary in the mine) whereas H5N1 triggers alarm and can be better controlled by early culling.
So far 47,8000 samples from 1,000 farms and poultry markets have been been tested and only 39 have been H7N9 positive.