Market networks.

A study by Fourni� in PNAS and colleagues looks at risk and H5N1 and live bird markets (LBMs) in North Vietnam. Such sites implicated in H7N9's diverse and rapid spread in South East China, notes the interconnectivity between markets using social network analysis. Some act as hubs, spreading poultry to others. Infection in a hub will more effectively spread virus to outlying nodes. 

A similar, older study of Southern China LBMs provides insight into the number of sites affected by hubs and the vast distances travelled by poultry (with the help of humans and vehicles) in this area. It unfortunately doesn't cover the South East, the H7N9 hot zone, but I presume the pattern can be extrapolated. The 2011 study, by Martin and colleagues, focuses on markets in the Yunnan, Guangxi and Hunan provinces. It shows travel on one of many routes extending 2,500km (Yunnan to Shandong) and the importance of risk reduction measures (for H5N1) including daily poultry cage cleaning and disinfection and manure disposal/processing.