Nonnative earthworms not all good for birds

Who'd ever think that earthworms are anything but bird food and fish bait?

Well, turns out earthworms also are a problem for forests and the birds that nest there.

It's because we have no native earthworms in Minnesota. All of the wigglers you dig from your garden or skewer on your fish hook are foreign invaders.

They didn't get here by themselves, since their own travel is restricted to a few feet per year.

We did it. Worms arrived with soil and plants brought from Europe, or dumped when foreign freighters on Lake Superior unloaded ballast.

If Minnesota once had its own earthworm species (unknown) they were scraped out of the state by the glaciers that plowed through most of Minnesota (and Wisconsin) thousands of years ago.

The invader worms can cause problems for certain bird species. We have forests with spots currently earthworm-free. Birds do better there. Actually, forests without earthworms do better in general.

Some years ago I toured a piece of land near St. John's University in St. Joseph, Minn., with people who were evaluating land for addition to the state Scientific and Natural program.

Walking through the woods, we stopped for a moment to look at the forest floor. It was mostly bare hard-packed dirt. Missing was the duff, the decaying vegetable matter you would expect to find.

Leaves that had fallen from trees the previous fall were missing. We happened to see a survivor leaf disappearing into a worm hole. Down it went. It was a strange sight. An invasive worm species was preparing dinner.

Studies by forest managers have determined that at least seven nonnative earthworm species have been introduced here so far. They cause loss of tree seedlings, wildflowers and ferns.

What does that mean for birds? Some of these plants are important to nests of hermit thrushes and ovenbirds. Populations of these species are lower where invasive earthworms are found.

Studies of this impact on birds were made by Scott R. Loss and Robert B. Blair. Loss at the time was a biology graduate student at the University of Minnesota, and now is an assistant professor at Oklahoma State University. Blair is a professor/Extension specialist in the Department of Fisheries, Wildlife and Conservation Biology at the University of Minnesota.

The pair wrote in a series of research papers that the worms are causing "substantial changes" in two Minnesota and Wisconsin forests where research was done.

"Bird surveys indicated significantly lower densities of ovenbirds and hermit thrushes in relation to worm invasions" at survey sites, they said.

They said there is "strong evidence that the presence of invasive earthworms on forest floors in northern hardwood forests affects populations of ground-dwelling songbirds."

The finding was most definitive for ovenbirds.

"These results indicate habitat changes that are strongly associated with earthworm invasions have biologically significant effects on ovenbird nest survival.

"Not only were fewer ovenbirds present in earthworm-invaded forests, but the birds that built nests in locations with high sedge cover and thin leaf litter — habitat attributes that are associated with earthworm invasions — appeared to have lower probability of nest survival," the research report said.

All that can be done about this is to stop human distribution of these worms.

Fishermen are the crux of the worm distribution problem today. All worms sold for fishing, including night crawlers, are invaders. They should not be tossed aside when the fishing trip ends, not in the water, not on shore. They belong in a trash can.

This is not a regional problem, by the way. It is countrywide, although there are native earthworm species in other parts of the country not affected by glaciers.

Cats, cars, windows, climate — of all the things birds must contend with, we add what many of us considered to be bird food — earthworms. How ironic.

Read Jim Williams' birding blog at startribune.com/wingnut.