Cooperative Fish and Wildlife Research Units Program: Pennsylvania
Education, Research and Technical Assistance for Managing Our Natural Resources

Since its arrival in North America in 1999, West Nile virus (WNV) has had unprecedented adverse effects on the health of native birds across numerous taxa. In Pennsylvania, WNV was first documented statewide in 2002, with concurrent and precipitous population declines in ruffed grouse (Bonasa umbellus). Recovery of grouse populations has not since been observed. Infected grouse may experience mortality rates as high as 70% or greater. The ongoing influence of WNV can be seen in statewide population indices such as hunter flush rates and late-summer brood sighting survey data. Recent analysis indicates that WNV and availability of young forest on the landscape synergistically influence colonization, persistence and extinction of local grouse populations. Susceptibility of ruffed grouse to ongoing cycles of high-WNV prevalence has important implications for managing this species. Monitoring vector species abundance can assist in understanding areas of the landscape that pose a risk to sustaining grouse populations.