Bird Diversity and the West Nile Virus, Part 2

A few months ago, I wrote about a study that found that greater bird diversity reduces the chances of human infection with the West Nile Virus. Now a second article has reported the same finding with some of the same data (e.g., the Breeding Bird Survey) as well as some field work of their own.

Allan and numerous graduate students began the research five years ago as they just entered graduate school and the topic of West Nile Virus was just beginning to receive lots of attention and the ecology of the organism hadn't been studied much. They identified a variety of field sites, both urban and rural, with their base of operations at Washington University's Tyson Research Center, a facility 22 miles west of St. Louis comprised of 2,000 acres of woods, glades and prairie.

They performed bird surveys at the sites, put up a variety of mosquito traps and studied different mosquito species and their ability to transmit the virus. Using kits provided by the Center for Disease Control, they tested the mosquitoes and found three positive pools.

"The infection rates are actually remarkably low, with maybe one in 1,000 carrying WNV," Allan said....

According to the researchers, they found that diversity is important both in the number of species and their proportions to each other. It does little good to have ten species in an area if two or three predominate. The ones most likely to predominate, like grackles and robins, are also the most likely to carry West Nile Virus and spread it to humans via mosquitos.

Protection from disease is yet another reason to protect wildlife diversity.

Bird Diversity and the West Nile Virus

Last week, PloS ONE published an interesting paper that argues that a diverse bird population lowers the human risk of contracting the West Nile Virus (WNV). The study compared WNV infection rates in sixty-five pairs of neighboring counties, in which one county had human WNV infections and the other did not, while both had infected birds. (One of the pairs included my home county, Middlesex, and its neighbor to the west, Somerset.)

The paper measured the inter-county contrast in human infection rates against the contrast in various measures of avian biodiversity. (Bird population data was derived from the Breeding Bird Survey.) It found that greater bird diversity correlated with fewer human infections. Avian biodiversity explained about 30% of the contrast.

The reason is something called the "dillution effect," which was first observed with Lyme disease transmission. The mechanism for the dillution effect is unclear; none of the proposed mechanisms matched up with the data in this paper's analysis. However, it seems to be related to the greater relative abundance of species that are poor hosts for the disease compared to species that are more susceptible. In the case of WNV, highly-susceptible species such as crows and robins tend to predominate in areas of lower bird diversity, so the disease will have plenty of hosts and a greater likelihood of spreading beyond birds.

Here are a few interesting points from the analysis:

1. The analysis confirms that robins play a role in spreading WNV to humans. This has been reported in other studies, including one in Washington.
2. Unsurprisingly, corvids (crows, jays) are associated with human infections.
3. Surprisingly, finches also seem to be associated with human cases of WNV. I had not heard of finch susceptibility before reading this paper.
   
4. Passerines as a group appear to be resilient in the face of WNV outbreaks, at least in the initial stages of an epidemic. This seems to contradict some previous reports, but perhaps not. The authors suggest that many passerine species may be poor hosts.
   
5. Non-passerines seem much more susceptible to WNV than previously thought, so there may be many susceptible host species outside of Passeriformes.
   

As Mike wrote, studies like this provide us with a self-centered rationale for maintaining a diverse avian population. When we promote wildlife conservation, we are also promoting our own health.

From my own persepective, it may support conserving wildlife within cities as well as outside of them. Many environmentalists and urbanists argue that urban parks should not be wildlife habitat and should be for people instead. (I do not think those goals are always in conflict, but the argument exists.) This paper provides an example of how increasing local biodiversity can be beneficial to humans in concrete ways. In the case of WNV, the dillution effect works independently of relative urbanization. This suggests that perhaps a diverse wildlife population should have a place in urban planning.

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Blog Note: I will not have internet access for the next few days, so I have prepared several posts to publish automatically. (If you do not know how to schedule posts, see here.) Though the blog will have the regular posting schedule, I will not be able to respond to comments or email until later in the week.

West Nile Virus Hurting Local Bird Species

A new study reports that several local bird species have declined since the West Nile Virus was introduced in 1998. The study included the American crow, an obvious candidate given news coverage of the disease. Other species affected by the virus include Carolina chickadee, tufted titmouse, blue jay, American robin, and eastern bluebird. (Oddly enough, fish crows do not seem to enter into the coverage, even though the other local corvids have been affected.)

West Nile Virus arrived from Uganda in 1998 and subsequently has been spread by mosquitos. It was first detected in New York City and slowly spread across the country from there, as far as the Pacific coast. While the disease has received a high profile from several early fatal human cases, its primary effects have been among bird populations. The study used data from the Breeding Bird Survey to measure populations before and after the virus was detected. From the Post:

After bottoming out in 2003 and 2004, house wrens and blue jays returned to their pre-West Nile levels in 2005, though it remains unclear whether they have developed immunity and whether those recoveries will last. Other species remain significantly down in numbers relative to what scientists would expect to be seeing had West Nile not arrived, based on trends over more than 25 years.

In the Northeast, for example, chickadees have dropped by 53 percent and the Eastern bluebird is down 44 percent. In Maryland, American robins took an especially large hit, with the virus apparently responsible for a 32 percent population reduction.

Whether these populations will bounce back, and how long it will take them to do so, remains to be seen. I think that they will, mainly because these are resilient species that have adapted well to the various other challenges that humans have thrown at them over the years. It may be a question of building immunity to the new disease and then rebuilding the population. However, the authors of the study, and the author of an accompanying commentary, raised the possibility of wider effects than just declines in these few species.

"The declines we see are probably a signal of a more serious ecosystem challenge that is having much broader effects than we're currently able to detect," LaDeau said....

For example, crows are important scavengers, clearing away roadkill and keeping competing pests at bay. And their penchant for eating other birds' young suppresses a wide range of other avian species.

"American crows are often considered a nuisance, but when the crows go, do we get more rats?" LaDeau asked. "What other scavengers come in, and what happens to the bird populations that are regulated in part by crows?"

The authors also raised the possibility of other diseases coming to the continent by a similar route - either by an insect or an exotic bird. When we think of introduced species harming to native birds, we have a tendency to focus on other birds, like starlings, or invasive plants, like garlic mustard or kudzu. It makes sense since we can see them. As it turns out, invasives that we cannot have caused a great deal of harm in a very short period.

Abstract from Nature:

• Shannon L. LaDeau, A. Marm Kilpatrick, and Peter P. Marra, "West Nile virus emergence and large-scale declines of North American bird populations."

Related posts:

• Dead Crow
  
• West Nile in the DC Area
• Crow Decline

(crossposted)

Birds of the Mid-Atlantic #26: Crows and Ravens

As Halloween is upon us, our thoughts turn to birds with spooky reputations. Owls, because of their nocturnal habits and otherworldly calls, are a Halloween staple. Vultures, with their preference for carrion, are a harbinger of death. Few birds seems as ominous as crows and ravens. Like vultures, their plumage is funereal black and they will eat carrion. Crows will gather in large flocks to roost; such roosts may hold hundreds or thousands of birds. Their reputation is such that the collective name for a gathering of crows is a "murder" and that for ravens is an "unkindness." These dark birds are sometimes symbols for impending death, as in Edgar Allen Poe's The Raven:

But the Raven still beguiling all my fancy into smiling,
Straight I wheeled a cushioned seat in front of bird and bust and door;
Then, upon the velvet sinking, I betook myself to linking
Fancy unto fancy, thinking what this ominous bird of yore,	70
What this grim, ungainly, ghastly, gaunt, and ominous bird of yore
Meant in croaking "Nevermore."

Crows and ravens are part of the family Corvidae. In North America, this family also includes jays and magpies in addition to its larger, darker members. Despite the familiar fable, members of this family are among the most intelligent of birds. They are known to use strategems to acquire food; jays, for example, will lure smaller songbirds away from their nests with imitations of hawk calls. Some studies suggest intelligence on the level of great apes. Crow intelligence is the reason that attempts to scare these birds from farms and gardens rarely work for long.

American Crow / Peter S. Weber (USGS)

The Mid-Atlantic region boasts three of these large, black birds: the American Crow, the Fish Crow, and the Common Raven. American Crows may be found throughout the region; these are the common crows whose caws may be heard in almost any neighborhood. Fish Crows are birds of the coastal plain. They may be found in marshes and along rivers; their distinctive call is a nasal nuh-uh. Common Ravens inhabit the western mountainous provinces; they may be found as far east as the piedmont, but rarely. They have wedge-shaped tails and emit a deep croaking sound instead of a caw. (This is not to be confused with a guttural croaking noise that American Crows sometimes make.)

Common Raven / US NPS

Unfortunately, our local crow population have been hit hard by the outbreak of West Nile Virus. American Crows seem to be particularly susceptible, but corvids as a group suffer a higher death rate when exposed to the virus. The graph below, derived from Christmas Bird Count results in the DC area, gives a sense of their decline in the past decade.

Because of recent worries about West Nile Virus and highly-pathogenic poultry flu in humans, crows again are becoming ominous signs. Only now, it is the dead crows rather than the living ones that are a cause for worry. With both diseases, birds are under a greater threat than humans.

Note: Audubon at Home has some good suggestions for making Halloween less wasteful.

Crossposted at Blue Ridge Gazette.

Tags:

• Birds
• Crow
• Raven

Dead Crow

This evening I found a dead crow while I was walking in my neighborhood. It lay in the middle of the sidewalk that runs along a small neighborhood park.

To see if I ought to do anything, I consulted the District government's page on the West Nile Virus. According to an FAQ, the District is no longer collecting dead birds for testing since the virus's presence in the area has been well-established. The Department of Health recommends disposing of dead birds yourself.

Dead birds act as a sentinel and dead bird surveillance is one of the components of the West Nile virus program in the District. The Department of Health has collected and tested sufficient numbers of dead birds to know that West Nile virus is endemic. The Department of Health is no longer studying dead birds. If you find a dead bird, please dispose of the bird yourself. To properly dispose of the bird, please follow this procedure:
* Wear protective gloves or use a plastic bag as a glove
* Place or wrap the dead bird in a plastic bag and tie the bag securely
* Dispose of the bag in an outdoor trash receptacle
* Wash your hands with soap and water
Remember, the West Nile virus is not transmitted directly from birds to humans.

Now I did not have the proper material to dispose of this creature myself, so I left it where it was. I imagine it will get flagged and disposed fairly soon anyway, since it is on a regular police patrol route.

When the West Nile Virus first appeared in the DC area several years ago, it appeared that crows were the main avian host for the disease. (The local crow population has declined significantly since the outbreak of the disease.) Recent studies, though, have suggested that the virus is more widespread, and that robins may be the most susceptible birds.

Related posts:

• West Nile Virus Hurting Local Bird Species
• Bird Diversity and the West Nile Virus
• BMA #26: Crows and Ravens
  

Tags:

• Crow
• West Nile Virus

West Nile in the DC Area

Researchers are studying the effects of the West Nile Virus on local bird populations and how the virus gets transmitted to humans. To do so, they have been setting up insect traps in certain parks around the Washington and Baltimore metro area to catch mosquitos. Birds at the same sites are being caught for blood tests. The results are interesting:

Robins, it turns out, appear to be taking the hit for humans, getting sick and dying as did thousands of crows that were infected in the first wave of West Nile virus after it arrived in North America. Thanks to the robins, humans who frequent the 26th Street dog park and similar areas have a lower chance of contracting the virus, at least in spring and early summer months. The reason? To mosquitoes, robins are far more tempting meals.

Then the scene changes.

"Robins begin to migrate south in late July and August," Kilpatrick said, "leaving mosquitoes on the hunt for blood from another source."

That source turns out to be Homo sapiens . The number of human infections with the virus shoots up come the dog days of August. Then it's mosquito vs. man or woman, instead of mosquito vs. robin.

It has been understood from the beginning that this is primarily a bird disease. Crow populations in certain areas have been devastated by the virus. Other birds have taken a hit as well. What I find perplexing is the migration explanation. Sure, robins do migrate, but their local numbers stay high at least through mid-October because the local migratory population is being replaced by robins travelling south from farther north. While the cause for the spike could probably use further explanation, I see no reason to doubt that the late-summer spike is real. So that should be extra reason to be careful about mosquito bites in late summer: do not keep standing water around your house; use insect repellent on bare skin or keep skin covered. (See this CDC advice on avoiding bites.)

Whatever you do, do not blame the birds! The mosquitos that carry the disease were most likely brought to North America on a very man-made airplane. You can only get West Nile from a mosquito bite, not from contact with an infected bird.

As an aside, the article mentions the following sites as "hot spots" for the West Nile Virus and for testing of birds:

Foggy Bottom near the Watergate Hotel is one of a number of urban hotspots for mosquitoes carrying West Nile virus -- and for Americanrobins. Other hot spots include the areas around the Hirshhorn Museum,the National Gallery of Art and the National Museum of Natural History.Not to mention Bethesda and Ridgeley's Delight, a posh neighborhood inBaltimore near Orioles Park at Camden Yards.

I am not sure that any of those sites qualifies as a "local bird paradise," as the title of the article suggests. Most of these are very compact areas with little space for breeding or foraging, and they are places that most birds avoid for that reason.

Read the original article in PLoS Biology.

Tags:

• West Nile Virus
• Birds
• American Robin

Crow decline

On Wednesday the Washington Post ran an story on the decline of crows in the mid-Atlantic region (thanks, Wildbird). The decline has been attributed largely to the West Nile Virus. While the virus has had little effect on human health, it has devastated crow populations, particularly among American crows.

An annual survey of breeding birds, organized by the Patuxent Wildlife Research Center and dating to 1966, shows a precipitous drop in Maryland's crow population in 2004. Birders in 57 locations tracked an average of 60 in 2001. The number dropped to less than 34 two years ago.

And in large roosts in Illinois and Oklahoma, three-quarters of the crow populations have dropped dead in a single year, said Kevin McGowan, a scientist at the Cornell Laboratory of Ornithology in Ithaca, N.Y. He likens the species' devastation to the human plagues of medieval Europe. In his freezer, he has the bodies of 30 crows he has known since they were nestlings, the oldest 13 years of age.

Local birders have noticed a difference.

Bird-watcher George Jett of Waldorf feeds crows and other wild birds. He says the once-large roost that inhabited woods near Routes 301 and 228 appears about a tenth its former size. Before the mosquito-borne virus arrived from the Middle East, he might have had 30 to 40 crows in his yard. "Now, I'm getting five or six," he said. "I hope the species learns to evolve with this. I enjoy crows."