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Did local birds ever recover from West Nile virus?
August 13, 2009

Dear Cecil:

When I first moved into my neighborhood near Graceland Cemetery, I was impressed by the number of birds I'd see out my window — robins, cardinals, and blue jays, and later crows. Then West Nile virus struck and birds virtually disappeared. Lately I realized I've been hearing birds singing again, so I guess they've staged a recovery. But there don't seem to be as many as there were years ago. What's the story? Is the bird population in permanent decline, and if so, what does that mean for environment?

— Betsy, Ravenswood

Cecil replies:

I'm not much of a birdwatcher, and can't tell a thrush from a grackle, but I don't mind admitting the silent summer of 2003 weirded me out, too. One year we had so many crows cawing in the neighborhood I felt like I was living on the set of a Hitchcock movie.  The next year — nothing. Not only was there no cawing, there was no birdsong at all. I'm not one to go looking for signs and portents, but when the birds went away, it took some effort to suppress the thought: surely this betokens the end of the world. Of course, that fall the Cubs pulled off yet another chokeroo, reassuring me the basic structure of the universe remained intact.

 Be that as it may, the bird situation bears watching. I'll do my best to unwrap a complicated story, but here's the gist: (1) The bird population has in fact mostly recovered from West Nile virus — but not completely. At least one species is just about gone. (2) Long term, meaning over a span of decades, lots of birds are just about gone, for reasons having nothing to do with the virus. (2) The disappearance of birds situation due to West Nile arguably has had an ecological impact, although it's probably not what you'd expect. Hint: it involves bunnies.

We'll handle this as a FAQ.

So, has the bird population recovered or hasn't it?

According to the people who should know — and Judy Pollock, director of bird conservation for Audubon Chicago Region, tells me we've got one of the most formidable birdwatching corps in the nation here — it mostly has. The Bird Conservation Network, a coalition of bird groups, maintains a detailed online count of birds spotted by its members, which includes charts showing the trend from 1999 to 2007. Bluebirds, since you mentioned them, have increased 12 percent annually during that time. Blue jays, the ones I used notice — they're the ones with the distinctive black, white, and blue markings on their heads and wings — have dropped 8 percent annually. But the species that has really taken it in the neck is the American crow. Its numbers have dropped an average of 26 percent per year — a 90 percent loss over the nine-year period. Birders occasionally spot a few, but it's fair to say that in the Chicago area the species has been basically wiped out.

That's mostly due to West Nile virus, Judy says. Researchers periodically capture birds and test their blood for West Nile antibodies; many species have a fairly high level. This suggests that while these birds get sick from the virus, it doesn't necessarily kill them. Crows, however, test low. Conclusion: if crows get West Nile, they die. Evanston was a hot spot for the disease, both in humans and in birds; Judy recalls that in 2003, at the height of the epidemic, crow corpses by the thousands littered the town's streets.

Not every species hit hard by West Nile virus was permanently devastated. Chickadee spottings, for example, dropped sharply in 2003, but recovered by 2005. They've since dropped again, but overall the species is down just 2 percent annually, which from a big-picture standpoint ain't bad.

Why's that?

Because even if we set the impact of West Nile aside, some categories of birds have gotten hammered over time, due chiefly to loss of somewhere to live. The experts categorize birds by four types of habitat — grasslands, shrublands, woodlands, and wetlands. Overall, the numbers of woodland birds such as chickadees have been stable in recent years. Not so grassland birds such as sparrows. Over the past 40 years, their numbers have plunged 90 percent as the grasslands where these birds nest have been built up. Also not doing so well are wetlands birds, with 14 species considered either endangered (facing extinction) or threatened (the level below endangered) in Illinois. Judy remains optimistic because of habitat conservation efforts in recent years.

So what's this about the bunnies?

I'm getting to that.  The northern suburbs have been overrun by rabbits in recent years. According to Judy, informed opinion attributes this to the plummeting crow population. A leading food source for the crow was the earthworm, which the crow would get by pecking at the ground. Also in the ground not far below the surface one finds rabbit warrens. The presumption is that crows, pecking for worms but finding baby bunnies instead, decided one protein source was as good as another. Thus fewer crows = more rabbits. Who knew?

 — Cecil Adams
Photo by Pat O'Neil

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