Effect Measure

Sick song birds

Influenza is primarily a disease of birds. Most emerging infectious diseases in humans are started out as diseases of animals, what are called zoonoses. We worry about zoonoses for that reason. It is one of the hardwired tendencies of any species to think of their own survival first — that’s natural — but humans are only one species amongst many. And while we worry about viruses we might catch from animals, the animals are also getting sick. It’s not just influenza we share with birds. Birds suffer from other diseases they can pass on to humans, too, and one of these is West Nile virus (WNV) infection, one of a group of insect borne encephalitis viruses that infect both birds and humans. Since arriving in North America in 1999, WNV has killed almost 1000 people but done considerably more damage to the bird population. A new paper in Nature tries to estimate how much damage.

West Nile virus or a similar disease could wipe out many of the U.S.’s backyard birds, profoundly changing some of the country’s most familiar wildlife and ecosystems.

That’s the finding of a new analysis of 26 years of data from the national Breeding Bird Survey?data that reveal the dramatic effects of the 1999 arrival of West Nile virus in the U.S.

Lead author Shannon LaDeau of the Smithsonian Migratory Bird Center and her colleagues found that species that thrive near humans suffered extremely high death rates from the disease.

Up to 45 percent of crows died after the virus arrived, with robins, chickadees, and eastern bluebirds not far behind.

Some of these populations had been increasing before the virus hit, which is a good indication that West Nile caused the declines, the authors write.

The disease may not completely wipe out bird populations on its own, the scientists add, but it is an alarming addition to existing population threats such as climate change and habitat loss.

“They’re our backyard species, and we haven’t been watching them as much as we’re watching the other species, because people consider them safe,” LaDeau told National Geographic News. (National Geographic News)

The virus is spread from bird to bird — and from bird to human — by mosquitoes. You might wonder how a mosquito can bite a feather covered bird, but there are small areas around the eye and elsewhere where the insect can and does gain access to the bird’s bloodstream. If that mosquito bites us, we can be infected, but this requires a “bridge species” of mosquito that bites both humans and birds and most mosquitoes don’t do that. Enough do, however, so 12,000 human cases of WNV have appeared in 44 of the 50 states and bird infections in every state except Alaska and Hawaii (map and 2006 case counts here). Human cases occur where there are infected birds because person to person transfer of the virus by a mosquito is thought not to happen because the level of virus in the human blood stream is not high enough.

But it is the effect on 20 common species of backyard birds that is the subject of the Nature paper. Disentangling the effects of the virus from the many other factors that affect bird populations is not an easy task:

To do so, they designed species-specific predictive models based on knowledge of the prevalence of the virus, exposure to mosquitoes and overall mortality for 20 different bird species, each species representing a specific combination of urban (human) association and susceptibility to the virus. The model was applied to 26 years of population data for six geographical regions to construct probability distributions for the expected abundance of each bird species in a given region before and after the arrival of the virus. (Carsten Rahbek News and Views, Nature)

The most affected birds, by their estimate, were the “peri-domestic” (backyard) species common to cities and suburban environments. One of the hardest hit was the American robin, but other species, including crows, chickadees, and bluebirds also experienced ten year population lows after the large outbreaks of 2002 – 2003. It was originally thought that crows and jays were the main victims of WNV, but that is probably because these birds are large and their corpses visible. We now know that many bird species, probably well over 100, are also infected, although many are apparently quite unaffected by the virus and may be a significant reservoir.

This is one of the first to gauge the effects of this bird infection on common species in North America, and even in the small number of species examined in the paper there are many uncertainties. WNV is a nasty human disease with a significant case fatality ratio and frequently debilitating after effects in survivors.

Maybe we don’t care that much about other species. But there are some good reasons why we should.


  1. #1 G in INdiana
    May 19, 2007

    I know my observations are purely anecdotal, but almost immediately after WNV hit our area there was a slight decline in some of the birds species mentioned in the above article.
    After that initial quiet spring and summer, the volume level is back up and increasing at my small Indiana farm. We lost several families of crows, blue jays, and American gold finches but all have bounced back in spades.
    The one species that has continued to decline is the ruby throated hummingbird. The year before WNV hit our area, we had over 75 hummingbirds at our feeders once breeding was done and the chicks fledged. Now we are lucky to have 20. This year something dreadful must have happened where they overwinter as we have only had a handful of birds show up. At the peak of the season, I would fill 15 feeders daily. Now I change the sugar water to keep it fresh instead of refilling empty feeders.
    I miss them terribly…

  2. #2 revere
    May 19, 2007

    It seems some of the hard hit species (e.g., Jays) have bounced back while others haven’t. It is very hard to unravel the many interacting factors here (land use, disease, weather, etc.), but WNV seems to have done a number of some birds. There are a number of other viruses that are of concern from the public health point of view, too, that affect the birds and other species. We aren’t alone in this world.

  3. #3 marquer
    May 19, 2007

    Our dominant local corvids are crows and ravens. Very rarely a jay or magpie. Upon learning that WNV was on the way, we began to keep a rough weekly census, figuring that the crows would be a marker for everything else (not least because they are large, loud and visible, in addition to being anecdotally vulnerable to the virus).

    Results of survey: we have seen no diminution whatsoever of the local crow population. They are located along a major flyway, so it is not as though they constitute an isolated and protected redoubt.

    On the other hand, European honeybee populations are way down from what they were three or four years ago, to judge by the number of foragers working the garden. Perhaps by as much as two-thirds. We don’t have Africanized honeybees here, or at least not yet.

    Bumblebee populations have also been greatly reduced. There have been two “false springs” so far in our area of Northern California, brief warm spells followed by tough patches of cold, fog and rain. I suspect that the young Bombus queens were fooled by these into starting their nests too early.

  4. #4 Melanie
    May 19, 2007


    That honeybee phenomenon is all over North America and spreading to Europe. There appear to be two causes, Colony Collapse Disorder, which is indeed mysterious, and varroa mites.

  5. #5 Lisa the GP
    May 20, 2007

    off topic–I hope y’all saw the NYT article about China poisoning toothpaste with diethylene glycol, as well as selling ethylene glycol labeled as glycerine to a variety of uses.

  6. #6 revere
    May 20, 2007

    Lisa: Yes, saw it. I have been away from keyboard more than usual, tending to family matters.

  7. #7 Interrobang
    May 20, 2007

    G — I haven’t noticed any decline in the number of hummingbirds about around here, which is mid-SW Ontario. I’m not sure this area has been all that hard hit by WNV, unlike some municipalities further to the south (Chatham, ON had a very bad outbreak, lots of dead birds).

    I’m going to tentatively rule out deep cold as a causative factor, because this entire area sits in a microclimate and is about one climate zone warmer than areas to the south. (Windsor, ON, is actually warmer on average than Pontiac, MI, for example.)

    What bothers me is the out-and-out fearmongering we’ve been seeing in the media around here about WNV. As far as I can tell, it’s not an extremely lethal disease, unless you’re very old, very young, undernourished, or immunocompromised anyhow. Do people find it scary because it’s an insect-borne pathogen? I don’t know, but I bet they’d freak out if they knew that malaria used to be endemic to this area… I mean, yes, take precautions (and anything that reduces the mosquito population somewhat is ok with me), be aware of the signs and symptoms, but would a little less “ZOMG OH NOES WEST NILE!!!!1” from the media be too much to ask?!

  8. #8 revere
    May 20, 2007

    Interrobang: If you get clinical symptoms (and there are a lot of subclinical cases to be sure) it is not a very nice disease. Not as bad as Eastern Equinve, but bad enough. It is an encephalomyelitis and many people have long lasting sequelae. That doesn’t mean we should haul out mosquito sprahying, which is often the first response. I am against aduliciding for WNV because I don’t see any evidence it does any good, but I have respect for this disease. It’s not a walk in the woods.

  9. #9 Susan Och
    May 20, 2007

    In the summer of 2002, we attended a family reunion on my husband’s side near St Cloud, Minnesota. The whole family was bitten by plenty of mosquitos, as is the norm in Minnesota.

    A few days later, the 14 year old daughter came down with a slight sore throat and a fever (102-104) that lasted a week. The 5 year old had the same fever, also for a week, but about 4 days later.

    As an experienced mom, this long lasting fever was something new and odd. I thought of West Nile. When I asked the doctors, they said that there was no quick test for WNV, so I should just watch the kids for complications. They both recovered with no ill effects.

    I figured that in a few years someone would start testing people populations to find out how many of us are carrying antibodies, and then I would be able to make a guess as to whether we actually had West Nile in the house or not. I didn’t think I’d still be waiting this five years later, but I guess it’s nice to find out about the birds.

  10. #10 bc
    May 20, 2007

    If you’re interested, an article was published recently on the long-term neurological effects of WNV, which may occur not only among those with WN meningitis or encehpalitis, but also among those who had only WN fever, .
    See http://www.journals.uchicago.edu/CID/journal/issues/v44n12/50140/50140.web.pdf

    It’s possible that it may even occur among people with asymptomatic infection (about 80% of those infected), we just don’t know yet. The veterinary pathologist Dr. Tracy McNamara, formerly of the Bronx zoo and first to identify the cause of the outbreak in 1999, tried to point out this risk when she found chronic brain inflammation in zoo animals who died of other causes and had not had symptomatic WNV.

    Her reward for her efforts was to lose her job and nearly destroy her career.

    There’s a lot we still don’t know about this disease.

  11. #11 marquer
    May 22, 2007

    Yes, I am aware of colony collapse disorder. We are in an area far from any crops which might be treated with neonicotinoids, so whatever is knocking down our local colonies is presumably not that.

    There was a single honeybee in the garden today. It left before I could get a good look.

    The bumblebees have increased in number, and we have clover out which they just adore.

    The comment about hummingbirds is interesting. We will add those to our bird census. Rather more difficult to track than are crows…

  12. #12 Kathryn Stillwell Burton
    August 6, 2007

    I am an associate of RAMSAR, an international group founded in Iranin 1972. it focuses primarily on wetlands and waterbirds and is partnered with Waterbirs International an dother groups around the
    world. Here is their take on the bird flu:

    Greetings. Ramsar STRP member David Stroud of JNCC writes: New guidance
    from the Scientific Task Force on Avian Influenza has summarised
    practical lessons learnt from outbreaks of infection by highly
    pathogenic avian influenza H5N1. A recent workshop brought together a
    wide range of international experience in dealing with different aspects
    of H5N1 infections. The conclusions and recommendations
    (http://www.ramsar.org/wn/w.n.avian_flu_aviemore_conclusions.pdf) from
    that meeting contain much useful guidance, as called for by Ramsar
    COP9’s Resolution IX.23 on avian influenza. The meeting condemned the
    continued misplaced practice of actively killing wild birds and
    destroying their nest sites and wetland habitats in response to, or in
    avoidance of, infection within a country, a practice which is contrary
    to the recommendations of many international bodies including the Ramsar
    Convention. Such approaches to the prevention or control of avian
    influenza are wasteful, damaging to conservation, and have no scientific
    basis. They may also exacerbate the problem by causing further
    dispersion of infected birds.

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