A new study reports that several local bird species have declined since the West Nile Virus was introduced in 1998. The study included the American crow, an obvious candidate given news coverage of the disease. Other species affected by the virus include Carolina chickadee, tufted titmouse, blue jay, American robin, and eastern bluebird. (Oddly enough, fish crows do not seem to enter into the coverage, even though the other local corvids have been affected.)
West Nile Virus arrived from Uganda in 1998 and subsequently has been spread by mosquitos. It was first detected in New York City and slowly spread across the country from there, as far as the Pacific coast. While the disease has received a high profile from several early fatal human cases, its primary effects have been among bird populations. The study used data from the Breeding Bird Survey to measure populations before and after the virus was detected. From the Post:
After bottoming out in 2003 and 2004, house wrens and blue jays returned to their pre-West Nile levels in 2005, though it remains unclear whether they have developed immunity and whether those recoveries will last. Other species remain significantly down in numbers relative to what scientists would expect to be seeing had West Nile not arrived, based on trends over more than 25 years.Whether these populations will bounce back, and how long it will take them to do so, remains to be seen. I think that they will, mainly because these are resilient species that have adapted well to the various other challenges that humans have thrown at them over the years. It may be a question of building immunity to the new disease and then rebuilding the population. However, the authors of the study, and the author of an accompanying commentary, raised the possibility of wider effects than just declines in these few species.
In the Northeast, for example, chickadees have dropped by 53 percent and the Eastern bluebird is down 44 percent. In Maryland, American robins took an especially large hit, with the virus apparently responsible for a 32 percent population reduction.
"The declines we see are probably a signal of a more serious ecosystem challenge that is having much broader effects than we're currently able to detect," LaDeau said....The authors also raised the possibility of other diseases coming to the continent by a similar route - either by an insect or an exotic bird. When we think of introduced species harming to native birds, we have a tendency to focus on other birds, like starlings, or invasive plants, like garlic mustard or kudzu. It makes sense since we can see them. As it turns out, invasives that we cannot have caused a great deal of harm in a very short period.
For example, crows are important scavengers, clearing away roadkill and keeping competing pests at bay. And their penchant for eating other birds' young suppresses a wide range of other avian species.
"American crows are often considered a nuisance, but when the crows go, do we get more rats?" LaDeau asked. "What other scavengers come in, and what happens to the bird populations that are regulated in part by crows?"
Abstract from Nature:
- Shannon L. LaDeau, A. Marm Kilpatrick, and Peter P. Marra, "West Nile virus emergence and large-scale declines of North American bird populations."