Community Science Reveals Lasting Impacts of West Nile Virus on Pennsylvania Bird Populations

A new study out of the University of Pennsylvania School of Veterinary Medicine suggests that West Nile virus (WNV) has had impacts on one in five Pennsylvania bird species, demonstrating how decades of community science data and mosquito surveillance can uncover the long-term ecological effects of wildlife disease.

Published in Biological Conservation, the study combined more than two decades of mosquito surveillance with bird population data collected by thousands of volunteer birdwatchers through the North American Breeding Bird Survey. The findings provide one of the clearest pictures to date of how West Nile virus has shaped bird populations across Pennsylvania since the virus arrived in North America in 1999.

“Understanding how disease affects wildlife populations is incredibly challenging because disease doesn’t occur in isolation,” said Brock Geary, PhD, a quantitative disease ecologist with Penn Vet’s Wildlife Futures Program and lead author of the study. “By combining large-scale mosquito surveillance with community science data, we were able to separate the impacts of West Nile virus from other factors such as habitat and climate, allowing us to identify the species experiencing long-term effects.”

To conduct the study, Geary collaborated with research institutions, government agencies, and bird conservation organizations to assemble extensive datasets spanning nearly two decades. The research incorporated WNV testing results from more than 9 million mosquitoes collected across Pennsylvania since 2000, along with bird abundance and population trend data for 155 species collected between 2002 and 2019 through the Breeding Bird Survey, a long-running community science program.

Using statistical models that accounted for additional influences, including habitat quality, temperature, elevation, and variances in bird abundance recording, the research team identified persistent WNV-associated population impacts in 31 bird species, representing approximately 20% of the species analyzed.

Many of the affected species were songbirds, including American goldfinch, yellow warbler, black-capped chickadee, tufted titmouse, and purple finch. Other species showing impacts included mourning dove, killdeer, sharp-shinned hawk, and northern flicker. The severity of the effects varied among species, with white-throated sparrows showing particularly strong negative associations, while American robins exhibited comparatively minor impacts.

The study also reinforced previous findings about where WNV is most likely to occur. Researchers found that warmer, lower-elevation landscapes with less forest cover consistently produced more WNV-positive mosquitoes. Combined with conditions such as stagnant water and reduced mosquito predation, these environments were associated with higher estimated virus activity along Breeding Bird Survey routes.

Although the findings significantly advance understanding of WNV ecology, the researchers note that community science data alone cannot capture the full picture. Some bird species are difficult to detect during standard daytime surveys, making additional monitoring approaches essential for understanding how disease affects more elusive wildlife.

“This study provides an important foundation for future research,” Geary said. “By identifying which species appear most affected by West Nile virus, we can begin asking why some birds are more vulnerable than others and ultimately improve wildlife conservation and disease management strategies.”

Ongoing research is examining how species traits and evolutionary relationships influence responses to WNV, building on the framework established by the new study.

West Nile virus, which is transmitted primarily by mosquitoes, was first detected in North America in 1999. While surveillance efforts have largely focused on protecting human health, this research highlights the value of these same datasets, when combined with community science efforts, in understanding the broader ecological consequences of emerging infectious diseases.

Lead author Brock Geary is a quantitative disease ecologist at Penn Vet’s Wildlife Futures Program.

Principal Investigator Roderick B. Gagne is an assistant professor of wildlife disease ecology in Penn Vet’s Department of Pathobiology, and he is an associate director in the Wildlife Futures Program.

Geary et al. study: Brock Geary and Roderick Gagne, Penn Vet; Adam C. Smith, Environment & Climate Change Canada; Lisa Williams and Sean P. Murphy, Pennsylvania Game Commission; Brandon P.M. Edwards, Carleton University, Canada; Nicole L. Michel and Timothy D. Meehan, National Audubon Society.

The work was supported by the Pennsylvania Game Commission.