A new study in Proceedings of the National Academy of Sciences indicates that West Nile virus (WNV) is killing millions of birds every year in North America, but few clear patterns are yet visible.
West Nile Virus (WNW), which arrived on this continent about 16 years ago from Africa, is carried by mosquitoes. It can infect and kill people, but birds are its preferred host. Because North American bird populations had not previously encountered WNV, they had not developed any resistance to it. Therefore, the virus spread across the entire continent in only five years, leaving millions of birds dead. Earlier studies had shown that other factors, such as climate and habitat, influenced the virus's effect on various bird species. For instance, it was found that birds in urban environments seemed more likely to contract it, for reasons that are still unknown.
A team of scientists analyzed 16 years of data collected from 1992 to 2007 at more than 500 Monitoring Avian Productivity and Survivorship (MAPS) stations across the United States. (MAPS data after 2007 weren't included because they have not yet been processed.) Using this information, the scientists were able to determine whether and how the virus first affected various bird populations and whether their numbers have recovered or are still declining. They studied a quarter-million birds from 49 species, focusing on adults.
Twenty-three of the species studied were negatively affected. Some initially suffered huge declines. For example, Red-Eyed Vireo (Vireo olivaceus) populations dropped about 29% in the year they first encountered the disease. As expected by disease ecologists, their numbers subsequently recovered. Ryan Harrigan, an infectious disease biologist at the University of California, Los Angeles (one of the study's authors) explains, “Everyone builds up immunity, and the impact tends to wane.” Eleven of the 23 affected species in the study experienced this type of recovery.
But the other twelve species were not so lucky: their populations are still declining for unknown reasons. For example, populations of Warbling Vireos (Vireo gilvus) dropped only 8.7% when they contracted WNV. But instead of recovering, they've continued to drop every year by roughly the same percentage.
To try to understand why some species overcome the disease better than others, the researchers compared their habitats. Here, too, results were mixed. Ten species, including the Spotted Towhee (Pipilo maculatus) and Song Sparrow (Melospiza melodia), did not show significant losses among adults, but eleven other species in these same urban areas did. The scientists also studied whether closely related species were affected similarly by WNV. Again, the pattern was mixed. Researchers are now looking more closely at regions where certain species are still dying to see whether they can spot the reason. And they have no idea why WNV did not affect at least three species, the black-capped chickadee (Parus atricapillus), American robin (Turdus migratorius), and house wren (Troglodytes aedon). Meanwhile, Staffan Bensch, an animal ecologist at Lund University in Sweden, wonders whether the virus might even be benefitting some songbird species by killing off jays and crows, which prey on them.
One further area of concern is that the virus may be causing particular harm to birds with smaller populations and ranges--that is, endangered birds. No answers are presenting themselves at this time. The researchers say that finding them will require more long-term data of the type collected at the MAPS stations. Ultimately, they hope that understanding the behavior of WNV in bird populations will help us understand its behavior in human populations.
For further reading, check out the study: http://www.pnas.org/content/early/2015/10/27/1507747112