It’s the transport of food & poultry that is the true culprit.
Read the press release from the British Ornithologists’ Union
“Human commercial activities, particularly those associated with poultry, are the major factors that have determined global dispersal.”
The global network of migration routes seemed to hide the real vectors – without strict health controls – of the exchanges of poultry, the more likely mechanism for disease spread. [link to British Ornithologists’ Union]