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The ecology of illegal aliens

Bill Meredith

“When the colonists came to the Americas, they brought their diseases with them.” …probably first said by Jared Diamond in Guns, Germs and Steel; plagiarized by many other sources on the internet.

There was something romantic and heroic about history as it was taught when I was in school. We learned about Columbus discovering America, brave colonists surviving winter hardships in New England and fending off savage Indians at Jamestown, and small bands of conquistadors conquering the mighty Aztec and Incan empires in Mexico and Peru. It was my favorite topic; my teacher was a good storyteller, and it made a great story. The trouble was that large amounts of the truth were left out.

The real story of the treatment of Native Americans by English and Spanish colonists began to reach the public through movies and television in the 1960s. Atrocities make good visual drama, and incidents like Wounded Knee began to get equal time with Custer’s Last Stand. But it was not until the late ‘80s that it began to be widely known that the greatest weapon the Europeans had in their battle to win this continent was the array of diseases they brought with them. Smallpox, scarlet fever, measles, typhus, syphilis, and cholera killed many times more Indians than bullets and whiskey.

Given the state of medical knowledge in the 16th and 17th centuries, it should come as no surprise that the Europeans brought diseases to the New World. And it is not just humans that transport diseases when they move; it is a common observation in ecology that animals bring their diseases with them when they immigrate into a new area. You don’t have to go far to see examples. For the past several years, as I watched house finches eating sunflower seeds from the feeder outside my kitchen window, it was not unusual to see individuals whose eyelids were encrusted with a wart-like material. Birds with this condition are easy to spot even if you can’t see the eye; they are lethargic, their feathers are not well groomed, and they are less alert when cats or hawks are around. Mercifully, most of them are killed by predators before they become completely blind and starve.

House finches are native to Central America; the first one I ever saw was when I was studying at Arizona State University in the summer of 1962. They are like English sparrows in size and behavior, roosting, nesting and feeding in flocks. The females are brown and drab, but the males have an attractive reddish orange on their head and back. They can carry a tune much better than English sparrows; their song is a varied, melodious warble that goes up in pitch at the end as if it were followed by a question mark. It was this coloring and song that got them in trouble; people used to catch them and sell them as caged birds. Then, in the 1940s a law was passed which made it illegal to sell wild birds. The finches that are now one of our most-common local birds are believed to be the descendants of a small number that were released from a pet shop in Tennessee in 1944, so they really are illegal aliens.

I was playing golf with John Richards at Carroll Valley some time around 1980 when we heard an unusual bird song. John knew birds much better than I, and he was the one who recognized it as a house finch. In the following years there was a population explosion of house finches throughout the mid-Atlantic states, and they became one of our most-abundant songbirds. Then in the mid ‘90s people began to notice specimens with eye infections, and over the next few years the finch population declined rapidly. The cause was found to be a micro-organism with the awe-inspiring name of Mycoplasma gallisepticum. The mycoplasmas are a group of very small bacteria-like organisms; they do not have cell walls like the true bacteria, so they resemble tiny blobs of protoplasm. This one was known to cause respiratory infections in chickens and eye infections in turkeys; its species name comes from the Latin “gallus,” which is the scientific name of the chicken, and “sepsis,’ which means infection. It is not known whether this organism jumped from domestic poultry to finches or if the finches already had it when they arrived here. Whatever the origin, the dense populations of finches and their habit of communal feeding made it easy to transfer the infection. Lab studies suggest that the eye infection itself is not fatal; house finches in captivity have been known to recover from it. But in nature, infected birds have little chance of surviving. I haven’t heard of humans being infected, but no one should take the risk of handling sick or dead birds.

This winter there have been unusual numbers of purple finches at my feeder. The purple finch is a northern species which migrates south as far as the Gulf States each winter; this year a lot of them stopped in Emmitsburg, and they have been in my yard every day. Superficially they look like house finches, but the males are colored more magenta than orange and the females have a distinctive white stripe through the eye. About a month ago I noticed that a few of the purple finches had contracted the eye infections; apparently the mycoplasma organism has jumped across species lines again, this time from the alien to a native species.

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