Health officials in Canada are very busy these days. They are placing chickens at fixed points all along their 2,500 km border   with the United States of America. It's not a joke, nor have the Canadians gone mad. They are using these chickens to see if the West Nile virus   is still around. The virus infects birds, so the chickens have a chance of catching the virus and fall ill. The West Nile virus is spread among humans. It killed seven people in New York last year. Countries around the world are realizing that it may just be possible to stop certain kinds of people from entering their land. However, it is very difficult to stop viruses traveling from one end of the earth to another. When they travel to new places, they get used to the environment very easily and sometimes start destroying the local plant and animal life. These biological polluters are called smart polluters. These smart polluters can be carried across borders of countries unknowingly. Just as we humans are travelling across the world more often than earlier, these biological polluters have also started journey much more. They slide into airplanes through their wheels. They dig into objects that travelers may be carrying from one country to another. There are some good examples of how these polluters work. The water hyacinth   of South America is blocking lakes in China and Africa. Tree snakes from Papua New Guinea are busy eating up some birds in the country of Guam, which breaks its natural balance. That's why the customs   officials in many foreign countries prevent people from bringing in a small plant, or an object made of wood that is in its natural form. The customs officials have these rules because these varieties   of plants are special to certain places. The plants have the power to spread new illnesses among native plants and animals. Biological polluters always create problems in places where they do not belong. They could be special varieties of plants, bugs or even animals. Why are Canadian officials placing chickens along the border?
To help clean up the border. To see if the virus is still around. To protect the chickens from falling ill. To make sure that the birds can find food.