Pigeons would be the most likely means of introducing the virus into the mosquito and bird population, IMO.
It is common for people to own pigeons. Anyone handling a sigificant number of crows, blue jays or any other species would draw attention from neighbors.
The 1999 Introduction of the West Nile Virus to North America
West Nile has been isolated from pigeons in Egypt, turtledoves in Turkey and wild bird species in Borneo, Cyprus, and Nigeria. Crows experimentially infected with WN showed high mortality. In nature, some crows must survive infection since antibody-positive individuals are often captured. A domestic pigeon infected with WN and showing clinical signs of illness was captured in Egypt. Domestic pigeons are abundant in urban settings and may prove to be particularly important in epidemics such as that observed in NYC during the summer of 1999