The changing natural world at our doorsteps | Illustration and text by Patterson Clark
January 3, 2012
American crows: Rebounding from a lethal epidemic
American crows have tight-knit families during the breeding season, with adult daughters and sons helping their parents raise the yearly brood. Even after young adults finally leave home to start their own families, they will occasionally fly over to visit the folks or sail off to catch up with a sibling. But close kinship breaks down when a crow family joins a large winter flock.
Similar to a human clan arriving at a mall for a post-holiday sale, once crow parents and offspring enter a flock, they split up and do their own thing. "They arrive together, but they don't stick together," says Kevin J. McGowan, an ornithologist at Cornell University. Crows resume family ties when the flock disbands in the spring.
Winter flocks may find advantages to living in cities, where they are safe from hunters and they find food in landfills and mall dumpsters. But crows do like making trips to the countryside. "They are always on the lookout for waste grain in agricultural fields," says McGowan, "and they will eat any animal they can subdue." Their meat menu for the winter includes mice, road kill and overwintering corn-borer caterpillars that crows peck out of the bases of cut corn stalks. Crows will also visit bird feeders, says McGowan, "but they are nervous about that."
Some of the flock's older birds are survivors of an epidemic that wiped out almost half of the local crow population in 2004, when West Nile virus arrived in the area. The virus, spread by certain mosquitoes, has mutated to become especially lethal to crows, killing every crow it infects.*
But after the epidemic, crow numbers stabilized and even seem to be recovering. Scientists don't see much evidence that crows have built an
immunity against the virus, says McGowan, suggesting that conditions may need to be just right before another epidemic occurs. "There's always a potential for a flareup," he says, "but we don't have the slightest idea what could trigger it."
* In contrast, most human infections go unnoticed, and mortality is much less than 1 percent.
SOURCES: Centers for Disease Control and Prevention, Audubon, Penn State University, USGS Patuxent Wildlife Research Center