Structure of the metapopulation model. H _y,a,z : proportion of birds in age class a, health state z of population y; V _x,z : proportion of vectors living at location x that have the health state z; μ _y,a : daily mortality rate of birds in age class a of population y; μ _v : vector daily mortality rate; λ _y,a : force of infection for hosts in age class a of population y; Tv: duration of viraemia in hosts; Tg _x : duration of gonotrophic cycle in vectors living at location x; Tx _x : duration of the extrinsic incubation period in vectors living at location incubation period in vectors living at location x; q _y,a : proportion of bites that occur on birds of population y and age class a; and p _y,a : proportion of viraemic birds in population y and age class a.