One of the Chicago area's most distinctive sounds has been eerily rare for more than a year. The unmistakable aw-aw, aw-aw that used to wake us at an ungodly hour on weekend mornings or rise to a deafening collective roar is almost completely gone. Even among birders, it's become nearly a triumph to be able to say, "I heard a crow today!"

Christmas Bird Counts in the Chicago region reported an 80- to 85-percent drop in the crow population last winter, according to Doug Stotz, conservation ecologist and ornithologist for The Field Museum. Famous local roosts in the western suburbs and in downtown Evanston once attracted so many noisy, messy crows that a Northwestern University student even started a locally famous, Web-based crow-poop alert. But from peak populations of more than 200,000 birds, the suburban roosts have been almost totally decimated. "I count crows on my way in [to The Field Museum] from Westchester, and I used to see 100 or more every day," says Stotz. "Now it's not uncommon to see no crows at all on my drive in."

Many researchers now blame the West Nile virus for the nationwide devastation to the American crow. At a Centers for Disease Control facility in Fort Collins, Colorado, eight crows infected with the virus in a laboratory all died within six days, scientifically establishing the virus' high fatality to the species. Further experiments there also showed that crows can pass the infection among themselves through saliva, feces, from consuming carrion, or by eating an infected insect.

In Champaign-Urbana, Sarah Yaremych, at the time a graduate student with the Illinois Natural History Survey (INHS) team at the University of Illinois, followed a group of 55 radio-collared crows in 2002 and found that as many as two-thirds of them died. West Nile virus was the cause of death in more than 90 percent of these cases. And a fall 2002 survey by Audubon-Chicago Region reported dramatic "dead zones" of both crows and chickadees after the influx of the virus, mapping out a patchwork pattern of mortality characteristic of West Nile.

Some research suggests that other factors also may have contributed to the crows' recent decline. A 2003 study by David Bonter and Wesley Hochachka studied Christmas Bird Count and Project FeederWatch data over several decades. The study showed that in certain areas of the country, crow populations have dropped by even greater numbers than at present in several years before the arrival of West Nile. One difference this time, the study says, is that crow, blue jay, and chickadee populations have experienced drastic declines simultaneously, something they have never done in the past. Statewide, the Illinois Department of Health collected 502 dead crows in 2002 and found 65 percent of them to be infected with the virus. The department says it has not yet collected enough dead crows in our region to prove a definitive link, however.

Whether other factors are involved or not, crows appear to be situated squarely in the path of the virus. Richard Lampman, INHS research scientist, says that the mosquito species Culex pipiens, a primary carrier of West Nile, has a "strong ecological and temporal overlap with the American crow." That is, these mosquitoes feed at the time crows are in their roosts, typically feed at the same height as the roosts, prefer to feed on birds, and are abundant in our area at the same time of year as crows are.

No evidence yet implicates crows in human infection. In 2002, the Illinois Department of Health stated that in areas of high crow deaths, more human cases of West Nile were reported. However, whether crows live long enough to re-infect mosquitoes that would then bite humans (the most likely avenue for crow-to-human transmission) has not been proven and might never be, says Dominic Travis, veterinary epidemiologist at the Lincoln Park Zoo. Mosquitoes are exceedingly difficult to track in the wild, and crows are so intelligent that researchers have difficulty trapping them in adequate numbers for study. However, the close correlation of human and crow infection carries weight: the Centers for Disease Control now bases its national West Nile surveillance system on crow deaths.

	Great eateries (garbage cans and Dumpsters) and entertainment attract crows to cities. Photo by A. Morris, VIREO.

The few crows we still see in this area might be families not yet infected or perhaps immune to West Nile, says Adam Ringia of the INHS. Typically, entire crow families succumb to West Nile once one bird has been infected. Over the past two years, crows have been regularly observed feeding others too sick to fly, almost certainly transmitting West Nile that way, either directly through saliva, or by providing more convenient targets for infected mosquitoes. Ringia points to a study by himself and other INHS researchers that found an antibody to West Nile in 5 of the 158 crows it examined. If immune, these survivors may endure further waves of the virus, provided it doesn't change, to repopulate the area.

The Field Museum's Stotz is optimistic that crows will make a comeback in the Chicago area, because it's a great place for crows to live. They came to cities in the first place to take advantage of the lights, in order to more easily spot their personal boogie man, the great horned owl, as well as other predators. Having great restaurants (Dumpsters, etc.) and entertainment (Dumpsters, etc.) doesn't hurt either. An address in the city and suburbs also protects these birds from crow hunting season, sixteen weeks in summer and winter when the bird is fair game.

"Conditions still exist that enabled them to thrive here over the past 20 years," says Stotz. "And since they produce four young each year, there is plenty of potential for a fast recovery. I expect to see them back in their usual numbers in a decade." Let the cawing begin again.