Many parasites make birds their home

In 1933 a man named Harold Peters contacted bird banders in the eastern half of the country. He asked them to collect the tiny bugs they found crawling on the birds they captured. He wanted examples of lice, mites, ticks and flies.

The pests on that long list are ectoparasites. All birds have them. The bird is where they live.

The 1933 survey was a very small and informal inquiry about how many different species of external parasites could be found on how many different species of birds. This was 82 years ago, but nothing has changed in that relationship.

Peters received 54 species of parasites collected (with tiny tweezers) from 75 species of birds. Lice dominated the parasite list: 41 different species. (And identification is not easy.)

Among birds common to our yards, a house wren carried 31 individuals of two parasite species. A downy woodpecker offered two parasite species, two of one, 11 of the other. A tree swallow had four each of three species; a chickadee, two of one species, eight of another.

Brown thrashers, robins and other thrushes, catbirds and blue jays were at the top of both lists — numbers of parasite species and individuals counted.

Multiple parasites per bird

My friend Tom, who taught high school biology, told me that all mammals have parasites.

I had some external parasites once, briefly. I poked with a stick at an old bird nest above me in a tree. I watched transfixed as hundreds of tiny, leggy creatures floated into my hair. They were lice or mites. I did not take time for identification.

Tom said there was no need to worry. "Most parasites need a particular host species," he said. The bird parasites weren't going to make homes on my head.

"It is possible to find several different kinds of lice on any bird species," Peters wrote in a research paper about his project.

Two kinds of flies are commonly found on songbirds, he said. "One or two species of ticks may be found on ground-feeding birds. Mites are found on most bird species," and the pest most commonly found on birds is lice, Peters said.

Different parasite species live on different bird parts. Peters described lice found on bird heads, necks and throats as "clumsy." Lice from backs and breasts were "rapid moving."

The good, the bad

Sometimes host and parasite both benefit from the relationship. Sometimes there is no cost to either. And sometimes the parasite harms the host. If in the latter case the host dies, the parasite has an obvious problem.

There are, of course, numerous problems caused for the birds by parasites. Nesting success is particularly affected. Young birds, those yet to fledge, are susceptible to the energy loss parasites cause. Parent birds suffering similar loss can be weakened enough to interfere with their ability to feed the young.

You could say that parasites are of general benefit by contributing to the mortality that nature requires to balance populations. Parasites do so by eliminating individuals showing weakest response to the infestation.

That's survival of the fittest.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at startribune.com/wingnut.