… adaptation of H9 viruses to land-based birds can lead to strains with expanded host range…. some bird species may act as a bridge in the generation of strains that can replicate more efficiently in mammals. In our particular case, quail provided an environment that improved the ability of a duck H9N2 virus to replicate in mice.

—¬†Hossain MJ, Hickman D, Perez DR (2008) Evidence of Expanded Host Range and Mammalian-Associated Genetic Changes in a Duck H9N2 Influenza Virus Following Adaptation in Quail and Chickens. PLoS ONE 3(9): e3170. doi:10.1371/journal.pone.0003170

(My emphasis)