Where have all of the birds gone?

StoryImage( ‘/Images/Story//Auto-img-11811533659153.jpg’, ‘Photo courtesy of www.duiops.net‘, ‘Blue jays are among a group of birds particularly susceptable to the West Nile virus.‘);

Those of us who are avid bird watchers have probably noticed that the numbers of different species of birds coming to our feeders the past few years has greatly diminished. I have made neither a qualitative nor quantitative study of the visitors to my feeding stations, but I estimate the numbers as 50 percent less than a few years ago. A recent study now supplies a possible reason why bird populations in our area have drastically declined in recent years: West Nile virus!

West Nile virus was first introduced into the United States from Africa in 1999, and, according to the Centers for Disease Control and Prevention, has infected 23, 974 people, resulting in 962 deaths. Illinois led the nation in 2002 with more than 800 cases reported, resulting in 62 deaths. A few cases have been reported from the Rock River Valley with the highly populated area of Cook County bearing the brunt of the virus’s assault.

This virus mainly affects birds and is transmitted to man when an infected bird is bitten by a mosquito, which later bites a human. In some cases, the disease in humans is quite mild, and one recovers rapidly from the slightly stiff neck and flu-like symptoms and has no idea he or she was infected with West Nile, but, for some reason, it may be quite virulent, and death may occur from damage to the central nervous system.

Many species of mosquitoes have been incriminated in harboring the virus, but the two most dangerous ones are the northern house mosquito, Culex pipiens pipiens, and the floodwater mosquito, Aedes vexans. The northern house mosquito is the most dangerous as it eagerly feeds on the blood of birds and human, while the floodwater mosquito prefers the blood of mammals but will take blood from a bird from time to time.

The Culex type will breed in almost any type of standing water and does not migrate far from its breeding site. Anything around one’s home that can hold water for about two weeks is a potential reproductive site. Clogged roof gutters, flower pots, wading pools, cans or bird baths are frequently utilized. It is imperative that during the mosquito season, you should inspect your premises and eliminate any standing water.

The floodwater type mosquito is a migratory form that usually moves from sheltered areas to attack you as night falls. Their eggs are laid in dry depressions that the female mosquito somehow knows will be flooded when it rains. As far as being important simply as a pest mosquito, the floodwater Aedes is by far the most prolific biter in the evening or if we venture into shaded areas during the day.

But, humans aside, the spread of West Nile virus has been far more important for several species of susceptible birds. The crow, blue jay, robin, house wren, chickadee and bluebird have been the hardest hit, hard enough to be significant.

The hardest hit, however, has been our old friend, the crow. A study by the Smithsonian institution in Washington indicates that nationwide about a third of the crow population has been killed by West Nile. Devastation of crow populations seem to occur in pockets, and the report notes that in Maryland, the crow mortality was about 45 percent, but in the Baltimore, Washington, D.C. areas, the loss was about 90 percent.

I frequently visit a friend in Baltimore and like to sit on the front porch of her home in the evening and watch the large flight of crows head for their roost in the densely wooded area on the grounds of the Maryland School for the Blind, which is nearby. The usual flock consisted of an estimated 50-75 birds, but the last time I was there, few months ago, the evening flock consisted of only about five crows. As a boy growing up in north Texas, I remember a favorite “sport” of some outdoors people was to go to a crow roosting area in the evening and see how many they could kill with a shotgun. I guess those sports people will have to find another way of sharpening up their skills with a scattergun.

As summer arrives, we should all be aware of the constant danger to us of West Nile virus disease. Mosquito breeding control around our homes should be practiced on a weekly basis, and skin should not be exposed to mosquito bites when we go outside in the evening. Exposed skin should be treated with the insect repellent DEET.

Though birds are having a rough go with West Nile virus at the present time, we should never forget that we are also targets for this devastating disease.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

from the June 6-12, 2007, issue

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