Article content continued

Then around 1994 it shows up in house finches. Why? And suddenly it is lethal to finches (though not to chickens). Roughly half the North American population of house finches is gone.

Second mystery: it affects other finches, too, but never enough to reduce the population. Goldfinches and grosbeaks can deal with it. And if a scientist takes the bacteria from an infected finch and tries to re-infect a chicken with that sample, it will cause no disease at all.

And a third: there’s a sudden evolution of a more potent strain of disease, which Cornell University bird scientist Wesley Hochachka calls “an arms race.”

It works this way: The original strain makes birds sick, but many finches survive and their immune systems learn to recognize the disease.

The bacterium reacts to the improved immune system, just as germs evolve to develop resistance to human drugs. It has formed a stronger strain and become able to re-infect a bird that fought off the weaker form.

It has actually done this twice — first in eastern North America and later in the west.

Neither side has really gained an advantage in the arms race. The birds have improved immune systems, and the disease has a stronger attack. In science-speak, it is more virulent.

And while the finches have not returned to being as common as they once were, they also are not losing numbers either. It’s a standoff.

Other bird species don’t seem bothered at all. “The bacteria seem to be coming into contact with all sorts of bird species, basically any bird that you would see in your yard or in forests nearby. We are actually picking up signs of antibodies” to the disease in many bird species — robins, wood thrushes and others. But there is no actual disease.

It’s similar to humans who have antibodies to West Nile virus, showing they have been exposed to the virus, but who have never had the disease.

Hochachka has some hints for birders. Clean feeders regularly, especially the types that have platforms where birds can walk, and put up another feeder if one is becoming too crowded.

tspears@postmedia.com

twitter.com/TomSpears1