AUTUMN 2007/VOLUME 21, NUMBER 4
West Nile Virus Disrupts the Family Lives of Crows
A deadly disease alters crows' complex societies
"Suddenly families were ripped apart," says Anne Clark. "Young birds, adult pairs, nonbreeding helpers, they all died." Clark, an associate professor at Binghamton University, and colleagues studied how West Nile virus outbreaks affected the complex society of American Crows after massive numbers of crows died from the virus in 2002 and 2003.
Their results, published in Ornithological Monographs (August 2006) showed that the outbreaks disrupted the social lives of crows, which frequently live in extended families with some members acting as helpers in raising young.
Coauthor Doug Robinson, a Ph.D. candidate at Binghamton University, kept track of crow families in a study area where coauthor Kevin McGowan had tagged and closely followed crows since 1989.
In 2002, more than one-third of the birds in the study died of West Nile virus during a two-month period. At first, no territories were lost or abandoned, and none changed ownership. The researchers concluded that the crows' social structure, with its extended families, was somewhat buffered from the effects of dramatic population declines, at least initially.
"But in the second year, 2003, we again lost over a third of the birds, and this time big changes happened," says McGowan, a research associate at the Cornell Lab of Ornithology. "Then we got movement of birds and changes in territory."
The pattern of movement was also unexpected. "One male lost most of his family, including his mate—and instead of attracting a new mate and staying on his territory he ended up joining another family nearby that had lost a breeding male," says McGowan. "There appears to be something about being in groups that's important to the crows."
When territories opened up because of deaths, other crows did not move right in. "Several families might have gradually encroached on a free territory, but none of them were quite sure they owned it," Clark speculates. "Crows have good memories. They knew their next-door neighbors and perhaps were reluctant to simply take over their territory. My term for it is the 'haunted house' effect—they responded as if they didn't want to go back to an area where birds have died."
Following the two peak years for West Nile virus, researchers noticed another unusual behavior. More females started carving out new territories from the territory in which they were born—a process called "budding." Usually it's the male crows that do this. Clark thinks the females, which normally disperse, may have found more breeding opportunities close to home after the virus swept through.
McGowan says it's luck that West Nile virus has not hit that hard again—adding the virus is not gone, just waiting for conditions to be right to bloom again.
"If West Nile were to hit hard in many more than two consecutive years," says Anne Clark, "younger birds would have to breed and they're not as successful as the older birds. We expect there would be a net decrease in the crow population that could snowball into a severe decline."
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